For all of the combat machine we see in real life, they are painted in colors to blend into the background and lower visibility in every spectrum. Except for when they are participating in a parade, their paintjob will always put "stealth" and "you didn't see anything" as the first piority. But I want to see a world with some--not all--but some of the machines to be painted in garnish and fancy color, regardless of whether they are tanks, fighter jet, or even huge bipedal mech piloted by some whiny kid that need to be fixed by two bright slaps. This world's technology, civil right, human right, and political wisdom is at least equal to modern era. But anything else other than these requirements, can change. So what change in history or human psychology will lead to flashy and anime-esque weapons a regular sight on battlefield?
It is Urban Camouflage
The International Style of Architecture, well known for is hundreds of different shades of greys, off-white, near-black, and beige was popular throughout most of the 20th century, but has fallen out of fashion. This color scheme is represented in what we today call Urban Camouflage. But in this drab palette's place, a new urban pattern is emerging. People don't want to see more concreate and stucco, they want color. So, in the 21st century, bright and highly saturated colors with outlandish patterns become the norm in cities across the world until 20th century urban camo makes you stand out like a business suit in a gay pride parade.
To adapt to this change in architecture, urban warfare weapons are painted in bright colors to break up their outline using Disruptive Camouflage in colors you would expect to find in the new urban combat arena, just like we do today with our dull coloration.
Fighting enemies who see differently
These days deer hunters tend to wear vests that are highly visible to other hunters (in order to reduce the chance of being shot accidentally) but are effective camouflage against deer (which are effectively red-green colourblind), as discussed in this question.
So if you want bright colours on the battlefield, fight an enemy that doesn't perceive colours the way humans do. Possibly the enemy is entirely blind in the human visual spectrum and "see" by sonar, radar, infra-red, ultraviolet or in some other way. In this case it makes much more sense for human combatants to make themselves highly visible to other humans in order to increase battlefield awareness, facilitate search and rescue and avoid friendly fire while making human combat vehicles "stealth" in the non-visual areas the aliens use.
Stealth is not feasible.
You have a fuel or power source, Element X, that provides tremendous amounts of power. Tanks can fire super-powerful railguns or particle beams, mechs can jump ten times their height, power armored troops can throw cargo containers like duffel bags. Any unit or vehicle using Element X is going to beat anything that isn't in a straight-up fight, by a huge margin. It's great...
...except that you can't hide it. The emissions or whatever else that Element X gives off when you use it for power are obvious, no matter what you do to try to hide. There's no sense having sophisticated camouflage when you're still going to be horribly obvious. You might as well be visible so that you don't spook friendly forces into accidentally attacking you.
This existed in real world naval warfare and was called Dazzle Camouflage
Per Wikipedia: "Unlike other forms of camouflage, the intention of dazzle is not to conceal but to make it difficult to estimate a target's range, speed, and heading. Norman Wilkinson explained in 1919 that he had intended dazzle primarily to mislead the enemy about a ship's course and so cause them to take up a poor firing position."
A vivid paint job with confusing patterns can disrupt visual range-finding and tracking. Dazzle camouflage only fell out of use during WWII when radar became common.
Why is Dazzle camouflage part of the meta again? Because in your setting, advanced radar jamming and other technologies have forced people to rely on visual tracking, just like before WWII. As a bonus it's probably going to be more effective against an AI's visual tracking than a human's.
Edit: I have learned that this principle has been applied to disrupt AI surveillance (eg facial recognition) in recent years under the name CV Dazzle.
It is required by treaties.
In our world we have a number of agreed upon rules of warfare, some of them fairly arbitrary. Shooting soldiers is accepted, but killing them with chemical weapons is not. Soldiers who don't wear uniforms can be treated as spies or terrorist and don't get the usual protection prisoners of war are entitled to. And so on.
It would be a perfectly reasonable rule that military vehicles are forbidden (under their equivalent of the Geneva convention) to use camouflage. That way, sneak attacks are less likely, there is a lot more clarity in terms of who is on which side and who is a civilian, and greater international trust is achieved.
Anyone trying to blend in with the background would be considered a terrorist.
A rule like this tends to benefit the stronger nations over the weaker nations - if you're the equivalent of North Vietnam trying to fight off the Americans, you have greater need of guerrilla tactics. But the fact that it benefits the strong nations is one of the reasons the strong nations came together to pressure everyone else to agree to this rule.
Combat machines in this world are stealthy because they have foes that can take them out. In a world where there are foes who can not take out a combat machine, a flashy one can be useful in some circumstances to warn people to not even try an attack, or remind them of the firepower that will be brought to bear in event of war.
Military uniforms took a distinct turn for the drab when it became feasible for snipers to be commonplace. The advantages of distinctive and easily seen uniform for close combat were trumped for the disadvantage of being a good target.
This can be something taking out long-range weaponry -- a chemical in the air that disables gunpowder -- or superior defenses to bullets and rockets that make close range fighting essential and bring back the advantages.
Too many solar flares
Once upon a time (15th - 18th century) the soldiers in the army used to wear uniforms with vivid colors, so the commanders could see where everyone was and understand the evolution of the battle with their own eyes. And that was needed because of the lack of instant communication between the elements of the army.
Actually I´m going to include here a question in another board that covers specifically that topic:
So, Let´s imagine a world where that same situation exists (lack of instant communication between the elements in the army). What could cause that situation in a scenario with the technology of our "modern era"? Well... I think solar flares could do the trick.
In an scenario with solar activity intense enough, and if that solar activity becomes "the new normal", radio communications could be impaired, and (even if you have modern technology) you will have to redesign your civilization and you may need to invent new ways to combat in the field, and eventually, a good idea could be to paint with vivid colors your combat machines, going back to those 15th-18th century strategies.
The actual warfare is being livestreamed and monetized to external consumers that treat them how RTS streams are watched today.
The additional money gained from this more than offsets any benefit to having camo paint, and soldiers and pilots may have their own unique paint jobs so that they are easier to pick out of the action by their fans and sponsors.
Either scanners and satellite images are powerful enough to completely eliminate the fog of war anyway, or maybe the consumer of the livestreams is powerful enough to enforce the forbiddance of one side viewing the streams from the other and are willing and able to use the banhammer on whichever side is found to be 'cheating'.
Stealth is impossible or very hard.
Power use, scanning tech, and all have improved to such a degree that stealth is impossible. Enemies can always see you, and so there's no point painting your combat suit one colour or another. Perhaps very rare experimental vehicles can hide, but the vast majority cannot.
Morale is important.
Nose art used to be very common, where pilots of fighter jets add art to their planes. Several studies and wars in your world proved that pilots and drivers with more art on their vehicles won fights. Their emotional connection with their vehicles meant they trained harder and spent more effort repairing them.
As such, every vehicle from an competent military has a paint budget to help encourage the pilot to take care of their vehicles.
They have active optical camouflage devices that can be turned on/off
Your machines have optical camouflage devices, like the capes in Ghost in the Shell. Once turned on, they are almost invisible to the eye.
So they can show as much bright colors as they want, when they engage they can just turn the device on..
You can even think of things like the chameleon cells at their surface, so they can even change colors on the fly.
Looking back at the history of war, we can note that the opposing sides are colourful when the need to tell friend from foe was more important than camoflage.
1a. Before the invention of effective long range weapons
A long range weapon lets you take out your enemy as soon as you can see him. If you are virtually guaranteed to see him before he comes in range of your weapon, then there is no point in him using camoflage. Where ranged weapons fail and you enter into the confusion of hand-to-hand combat, the important thing is to be able to tell friend from foe.
In the Napoleonic wars and the American war of independence, we see the Americans and the French in blue uniforms and the British in bright red. Although guns were available, they were slow and cumbersome to operate (hence the phrase "don't shoot till you see the whites of their eyes.") The bright red British uniforms persisted until the anglo-zulu war in 1879 (see also the film Zulu starring Michael Caine.) But by 1899 in the Boer wars the British were using khaki uniforms. The significant difference was that their Dutch opponents had guns whereas the Zulus didn't (except ones they had captured from Europeans.)
1b. when your world physically prevents the use of ranged weapons
Ranged weapons are of limited use in a thick atmosphere or underwater. Removing aircraft and fast ships from the equation makes camoflage of submarines of limited use. If you have so many submarines that it's important to tell who is who, distinctive "team colours" become desirable.
2. When you are winning and don't want to be hit by friendly fire.
During the last year of WWII, the Nazis were losing and Allied forces (re)-invading Europe dominated the air. The Allied planes were painted with black and white Invasion Stripes to make it clear which side they were on
3.(futuristically) when there are other means of detecting you
If your enemy can "see" you by other means (infra red night vision, sonar, potentially the electrical interference given off by your vehicle) it doesn't matter what colour your vehicle is.
A virtual requirement of practical 10 meter tall Mecha is that they have some technology that makes them virtually invulnerable to modern tanks, artillery and ground attack aircraft. That in turn would mean part of the reason you need camouflage would be gone - you don't have to hide from those threats.
In that case the situation may be similar to the early days of aerial warfare, where the only credible threat to a fighter plane was another fighter plane. There was a brief period where some fighter pilots saw themselves as knights of the sky, even going so far as notions of chivalry, and brightly and clearly identifying themselves to friend and foe alike. Some decorated their aircraft with distinctive, often bright, colour schemes and designs.
Perhaps Mech pilots develop a similar culture, recognising infamous enemies to challenge to personal combat by their individual paint jobs, or feeling the same fear as an allied airman over France in 1917 on seeing a red Albatross D V diving toward them, when they catch sight of a particularly notorious Mech coming over the hill...
The robots are unpainted
If your robots are already the size of mechs any additional weight starts to become a problem. Paint costs weight, so your manufacturers leave them unpainted. For a modern-day example you can look at how are beginning to leave rockets unpainted, but the base materials these rockets are made of is already orange - so the rocket itself is a bright orange. Metals can come in a wide variety of colors (metallic alloys can come in nearly all colors), so an unpainted robot could be quite vivid!
What few areas need to be painted are painted in a bright color, especially white, to reduce heat from both your mech's own exhaust and any incoming laser attacks.
Stealth is again the spirit of warfare
Gentlepersons must practice good war etiquette, the most important being "One must not use of stealth attack again the enemy forces". -- Some politician, maybe
Early on, a few civilizations decided to solve their conflict via small teams dueling championship between nobles to save on human lives and material costs.
A century or two later, they benefited from that agreement with an unprecedented golden age. Other kingdoms/empires that refused to adopt this practice started to lose on the economic and societal front. The terms diplomatic championships and diplomatic dueling are now used worldwide, and etiquette is starting to crystalize around the process.
A few more centuries, and it was adopted by most, and countries started to share the burden of armies maintenance needed against the few that still didn't practice dueling. Nobles started to attempt schools that trained from dueling and having one family member selected for diplomatic dueling is worth decades of honor.
When the enlightment equivalent happened: most of the philosophers take inspiration from those civilization successors, which promote the custom as a core principle of modern diplomacy. The practice starts to open to middle-class citizens, granting families increased social status. Countries start to have armies of only a dozen or so commando squads, but they are exclusively used for protection and intel, as using them is meant with heavy sanction worldwide.
Diplomatic championships are rare, but national teams are extremely popular, and friendly matches are part of the daily politic. Anyone can challenge a member of the national team, there is a process with fighting previous challengers or team members, and it's akin to any national sport in our universe.
Rules have evolved, to rate not only on technique but sportsmanship, style, and technological prowess. It's not rare for a country to hide a revolutionary technology for a few years during tense times, just so they can make a public demonstration of it during a diplomatic duel.
Matches are now rarely mortal as etiquette, on the winner and loser sides, is in favor of yielding or agreeing on a draw once the dominant duelist becomes apparent.
Teams are stars, life on the training center is broadcaster 7/24 by multiple stream-news companies. Engineers and scientists appear in weekly shows to speak at length of their design and kids are encouraged to be in support staff too.
Mechs made the show even more about panache and grace than ever, most mechs in public demonstrations are for peacocking during peacetime. Technologically advanced ones are disguised as peacocks so their testing would not be suspicious.
In the event when the aggressor's combat machines are way superior than the opposing force. They very well know that enemy's defense or combat machines are very primitive. Stealth does no benefit other than increase costs in this scenario.
If you see the airplanes that fought in the World War, they were colorful and proud showcasing who they where. These were testing times as countries didn't know everything about the other's capability like we do today, and attacking other lands were considered matter of pride and symbolism, which is why World War era planes were colorful display of bravery. Today intelligence gathering and RADAR monitoring capabilities are available to basically anyone, hence stealth has become the next obvious.
Just like in an MMORPG, these flashy mechs are your taunting tank. Their mere presence will attract enemy fire or enemy compliance.
A good army isn't composed of just a single unit which is Ctrl + C Ctrl + V'd into spam oblivion.
The enemy will try to attack the mechs instead of your:
- Weak yet deadly rogue
- Weak yet useful healer
- Weak yet deadly sniper
Fog Of War, difficult terrrain and no communication
Anime-mecha are really heavy (you see their steps shake walls few streets away, they left footprints in the normal road, stepping on car means the car is destroyed and they can throw cars far away by hands). More over they are armored so much, that classical guns, machine guns or even canons just scratch their painting, presenting ho danger to them. To move such montrum (and let it jump over buildings) you need terrible powerfull source of energy. Which needs little better than today technonogy, but say it is possible.
It just take its price - the new powersource is really noisy in el-mag spectrum. And shielding the noise off would render the unit nearly immobile. Forgot jumps, running or fast walking. In such way clasical tank design would rule. But if you left away the elmag shielding, you have anime-mecha, which can jump and run and fight had-to-hand or use also powerfull beam weapons.
Well, that also means, that it cannot effectively use radars and nothing near it cannot use radio. So communication is problem - in clear air it can use laser to line-of-sight receivers, but even smoke make it difficult.
Such mecha is not best design to fight on plain terrain, but it excel in terrible terrain, like cities, rock mazes or jungle, where the jumping capability simply make the change. So difficult terrain is it primary domain. Also that means, that line-of-sight is usually really short, say 3-5 long steps (in clean air). And if it shoots its weapons, then rocks start to melt, anything flamable will burn and smoke will be nearly everywhere.
Also it have to move really fast, as otherwise it would became good beacon for heavy rockets. But rockets in the sky can be shoot away with beam weapons and cruising low flying rockets could be avoided by taking corner, which makes the rocket hit the corner instead of you.
The only reasonable way to defeat such mecha is to use another mecha, or beam weapons with similar power-source (so at least car (or 6+ man) carried and with the same noise problems)
The war is then quick, in difficult terrain, short LOS and lot of smoke in air. Heat sources are plentifull - everywhere, where beam weapon fires, hits, rocket explodes, rock/concrete is melting - not good for targeting too. And both sides are deploying lot of units day or night - without actual communication channels and easy to mix together with enemy.
Now imagine, that you have beam weapon (or mecha or ...) ready, do not know, who is where, situation is changing each minute and it takes like 10 minutes to safely stop down your powersource to be able at least try communication with base and then another 10 minutes to start it again - rendering you for long 20 minutes defenless, but detectable.
And you hear some mecha aproaching from the other streat. It would be in effective range (including visibility) in like 15 seconds, then in 2-3 second it would be able spot and destroy you - would you fire at first possible moment, or would you wait and try to decipher friend-or-foe? Yes, you will fire, as you want to survive. Or you would be distinctively colored as well as the mecha and if the colors say "friendly" you would not fire and hope you would not be fired on. And the pilot of the macha is in the same situation - 8 steps ahead, behaind corner is one of many source od possible elmag noice - or it is just reflection from somewhere else, you have just few second, will you attack or not? If you could see familiar colors, you just continue your way, if you see enemy colors (or no colors at all), you will attack as fast as you can - especially when you are "some whiny kid that need to be fixed by two bright slaps".
So we have all powered units bright colored (well the mecha or may not may not find enemy, but surely have pass a lot of friendly units on way from its base and on way back. Having those units NOT firing on you greatly increases your chance to safe deployment and return, while fight with enemy would be still just fight - no big change, as you cannot stealth your way in and you will be attacked anyway, if you will not be fast recognized as "friend")
On the other hand "classical units" without power source would be still camouflaged and would avoid fighting mechas, as they cannot harm them anyway. But they can at least sometimes report to base, what they had seen recently. And you would also manage civilians, technology, recon, whatever the big units are not practical on.
And mechas would generally would ignore you, if some powered enemy MAY be near, as you are not direct thread to them. So you will fight only enemy not-powerd groups at your own level. Only when base will tell you, that there are nearly no powered alies in your area, then you would strat to care about enemy mechas, because that is the time, when they would have time to slow down, contact their not powered (but somehow informed) units and then go hunt you. If enemy owepowered your area, it is time to retreat.
As for civilians, they are mainly only collateral damage, victims of burning houses, falling buildings and such, but nobody would go after them to undeground bunkers and other areas of evacuation as long, as there would not be powered weapons installed. After place is conquared by powered units, it is occupied in convential way by army/police/similar units and mechas are only here and there to remember, that they have no chance, but play by rules of victor. And being brightly colored helps a lot to be recognized as symbol of victorius side.
Now we have all powered units brightly colored and probabely also in different styles and patterns, so they could easy recognize members of their attack group in heat of battle. Camouflage no way help them, nor your enemy.
Units are deployed into heat of battle as fast as possible (drop them from jet aircrafts is good option) and also as fast retreated to recharge/repair/regroup.
Battles on plain battlefields are more rare and done with totally different units and different strategies.
Shock and Awe:
If you've seen a WW2 movie where the Germans are attacking, you've heard the siren of a Stuka dive bomber. The sirens were added specifically to make these bombers terror weapons. The noise tells it's victims they are about to be blasted. Whistles were also attached to bombs to let people know that doom was coming. The same principle applies here. The point of a highly visible weapon is that you see it, know it's coming, and you can't help but notice it. Most likely, there's nothing you can do to stop it.
The same applies to the nebelwerfer a rocket launcher system who's sound was distinctive and easily noticed. Even thought he effectiveness of these rockets was actually low, the allied soldiers feared these rockets disproportionately because it was random and intimidating.
This is most effective in the initial phases of a war where you want to break the spirit of an unprepared enemy when they don't know how things were going to turn out. Think back to the earliest stages of COVID when the unknown of what is going on was more terrifying than the actual virus.
The special alloys that the combat machines use as a base react to the atmospheres of the various environments. Joints would be made out of a specific type of alloy, and armor sections made out of another type of alloy. Due to the source of pigment making not being made readily available in such environments (perhaps they don't or can't use petroleum as a fuel source) and due to frequent repair and lack of an advantage of color (due to radiation sensors) it is not worth having anything other than Inorganic pigments that are naturally produced on the alloys.
Organic pigments made from natural sources have been used for centuries, but most pigments used today are either inorganic or synthetic organic ones. Synthetic organic pigments are derived from coal tars and other petrochemicals. Inorganic pigments are made by relatively simple chemical reactions—notably oxidation—or are found naturally as earths.
One vital difference between "soldiers" and "warriors" is whether they fight as organised units or as individuals… contrast David and Goliath with the serried ranks behind them.
It doesn't matter whether the warrior wears only woad, full medieval armour or something more modern. The Red Baron was one of the last allowed to go the whole hog, but the decals and mottoes on tanks and planes in WWII or Vietnam and even striking nick-names and helmet decor a la Top Gun echo that style of fighting.
Swashbuckling bravado won't make it practical for Tank Girl or even Fury's tankers to charge about like Rooster Cockburn in True Grit and their buddies in Kelly's Heroes show heroic machismo more by its absence but look at the way Ripley's wields her cargo loader in Alien…
Depending on the writing, there's no reason anyone can't play "my weapon's bigger than yours…" with anything from a teaspoon to a battleship.
A paint that disrupts that target acqusition of an enemy that doesn't use our sight spectrum.
You and I, we see the world through the colors(unless your blind, or colorblind). This paint camoflages the target from whatever detections system THEY use, but not us.
Say, your fighting a robot army, and this paint scatter your radar signature and hides your thermal one.
Say, your fighting an army of monsters and looking at this paint gives them a headache.
Or any other detection system that don't use light. Whatever works