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In humans skin color varies slightly in shades of dark brown to a faded pink, which has the purpose of protecting the skin from sunlight. An idea used commonly by people beginning designing alien is to have their aliens have every possible color of skin under the sun, but in my mind this is kind of unlikely.

How can I explain why a species would have every possible skin color under the sun? Why would a race of species have orange as a skin color? What about red? Violet, Blue, Green, Yellow, etc?

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    $\begingroup$ Don't see what the problem is, even on Earth you can see animals with all sorts of different coloured skin & fur... $\endgroup$ – colmde Aug 25 '16 at 13:10
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There are numerous ways that evolution can select for specific traits. Evolutionary biologists often describe these sorts of pressures as The Four F's: fighting, fleeing, feeding, and... reproducing.

Others have already mentioned a number of situations describing these pressures in action. Camouflage is likely to be a defensive strategy (i.e. fleeing) though a well-hidden hunter might also find it useful (i.e. feeding). Meanwhile bright plumage might promote social status and/or attract mates (i.e. reproducing).

If you have a desired color in mind, simply imagine a scenario where this coloration would help that creature survive and have offspring by improving it's chances according to one of the Four F's.


However, whatever colorations are favored, those colors will inherently depend on what pigments are present in the organism's skin. Technically, it could alternatively depend on other phenomena (like the irridescence of butterfly wings) but simply assuming pigment==color is probably the easiest way to go, conceptually.

For humans (and lots of other creatures for that matter) the primary pigment is melanin (of which there are actually a few varieties). Biologically speaking, melanin is a pretty simple pigment made up of a few tyrosine molecules oxidised and strung together. These tend to absorb light from a variety of wavelengths, resulting in the mostly brown shades we associate with it for human hair and skin colors. Evolutionary, we all are starting from more or less the same melanin-producing ancestors, so even with a melanin mutation here and there over the eons that mostly just results in different shades of brown- maybe some more rust-colored here or blonde there, but generally from a similar color pallet. Melanin, as a pigment, isn't likely to mutate any blue or green hues anytime soon.

On the other hand, another well-known pigment, chlorophyll is a structurally rather different molecule that does a very similar job (although plants take things a step further, utilizing sunlight rather than just blocking it). At the forefront of the chlorophyll molecule is the Chlorin ring which is like a cage around a single metal ion. In plants, Magnesium is typically found at the center of this ring and that helps to make chlorophyll green. Porphyrin rings are very similar: one natural example is hemoglobin, the red pigment in blood, which cages an iron ion; and a second example is Phthalocyanine which is a bright blue commercially-used pigment which cages a copper ion. The reactions that actually trap the metal ion in the "cage" are typically pretty well selective at putting iron in hemoglobin and magnesium in chlorophyll. Things like altered pH conditions and/or mineral deficiencies, however, can sometimes throw the occasional odd metal into some of these cages- thereby altering the color. It is not too far fetched to think that some alien species may have evolved from an ancestor that had porphyrin-based pigments rather than melanin-based pigments. Furthermore, just as melanin mutations can lead to various hair colors, mutations could lead to preferential uptake of different metal ions and leading to different colors. Aliens don't need to be plants to have chlorophyll-based pigments.

(Bonus points: if pH and/or mineral intake can adjust color expression, then there's a high probability that well-to-do aliens might alter their diet to look more fashionable/appealing.)

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Structural colors

Not too far fetched; a single genus of bird (Ara; the macaws) has practically every color represented in the most vivid shade. Feather's colors usually arise from structure rather than pigment.

Structural color is not confined to bird feathers and butterflies' wings, however. The Golden mole's hair color is structurally-based. In a more fleshy-sense, if you've butchered beef you might notice some membranes have an iridescent property, hence its occasional name "silver skin" (not argyria, though that could be of-interest to you as well). Some folks at the University of Rochester's Optics institute got really into this.

If you want to get into some technical specifics, because the color is altered by the structure's spacing, I could imagine varying repeats in a gene (like a microsatellite inside a gene?) varying the size of a protein that could crystallize or somehow alter a distance in a heritable but also mutable way. Not sure how birds do it.


Inorganic pigments

Organic pigments (Fenix talks about most of them) are generally limited to browns, tans, and reds. Some can be blue-ish like indigo, or green like chlorophyll, but the preferred pigment of artists are metals in various oxidization states (e.g. lead white, cadmium yellow, red ochre, malachite, lapis lazuli/ultramarine). There is a whole group of metal-oxidizing/reducing enzymes in life here on earth, so why not elsewhere?

To get a rainbow, vanadium oxides range from purple to blue to green to yellow (see this video demo). If vanadium was an abundant element somewhere, something might be able to make use of it. Mix in some iron oxide (the red of blood), and you have a fairly complete range by altering the oxidization state/concentration of two elements.

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You could try having aliens with chameleon-like skin, that changes colour to match the environment. This might not match what you're looking for (which appears to be fixed skin colours for individuals), but it would be a place to start.

Failing that, you might try having an environment with significant divides between various climates, enough for the aliens to develop races like humanity did before they developed a near-modern civilization (at which point technology neutralizes most natural selection pressures, and all bets are off on where evolution might go). You would want to minimize the mixing of these proto-races before civilization develops, but this wouldn't be terribly hard to do; just look at Earth for examples.

Only this time, instead of having skin variation determined primarily by sunlight exposure as it was in humans, it might be determined by the colours of the environment for camouflage. Thus, you might have white-skinned individuals develop in the tundra and arctic regions, brown or green skins in the forests, green-yellow out on the plains, grey or black for any tribes that take to nocturnal hunting over extended periods of time, etc. If your aliens can fly, blue might even be an option to blend in with the sky. This won't necessarily give you a full spectrum of colours (I have a hard time seeing pink as an option), but it would give you a good variety.

Note that most mammals, to my knowledge, are limited to shades of grey or brown (stretching to black and white) for their fur, or perhaps even a little red. There are biological reasons for this, namely the pigment that gives fur its colour (melanin, I believe) only being capable of producing that colour range. If you want your wider colour spectrum, you're likely going to have to look at birds or reptiles.

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    $\begingroup$ an alien planet won't have anything like mammals,birds,reptiles or any kind of animal at all... they'd have a completely different kingdom $\endgroup$ – Charon Aug 23 '16 at 18:34
  • $\begingroup$ @渡し守シャロン not true, it bepends on the planets, assuming that the planet has an earth like environment, it will produce animals that are at least similar to earth animals in the basics. Some will have fur or scales, give live birth or eggs, etc. $\endgroup$ – TrEs-2b Aug 23 '16 at 18:38
  • $\begingroup$ it will produce consumer multi-cellular carbon based living organisms* unless you bring them to this planet there won't be animals, you probably won't even find the same kind of cells or building blocks, aliens might not even use proteins at all. $\endgroup$ – Charon Aug 23 '16 at 18:50
  • $\begingroup$ @渡し守シャロン That's possible, in theory. However, DNA has not yet been beaten out by any other biological structure for its purpose, proteins are a very effective structure, etc. If life forms on another Earth-like planet, it's plausible that it will have the same underlying chemical mechanisms. Evolution is a universal principle; the most successful traits are the ones that are carried forward, which would likely produce many life-forms similar to deer, bears, trees, and so forth on another planet. And of course, writers don't have the resources to invent thousands of new species for one story. $\endgroup$ – Palarran Aug 23 '16 at 19:00
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Fashion and indicators of social position.

Nations, clans, or tribes of these aliens have selectively bred for skin color over many generations, as a way of indicating families with high status, as a way to distinguish between castes, or as a way to emphasize that they are different from neighboring groups.

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  • $\begingroup$ This concept is blatantly obvious in old "Bollywood" movies, for example. $\endgroup$ – pipe Aug 24 '16 at 8:38
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Chromatophores might be an interesting answer to your question. Certain animals such as octopus have these cells which effectively grow larger or smaller in response to particular stimulus which changes their color similar to dithering with pixels. It's entirely possible for an alien species to have chromatophores which are red/green/blue, allowing them to be a number of visible colors.

Some animals, plants, and insects also have infrared markings which, while humans can't see naturally, others of their species can which drastically change what they may look like as well if you wanted even more flair.

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I have another version of John Feltz's answer. Whereas "fashion" is good idea, it might be completely artificial rather than natural ("bred"). (the question didn't state that it had to be an evolutionnary process, so I'll go with this as other answer already said a lot)

For instance, some do dye our hair multiple color, or do tatoos (that sometimes cover the whole body). It's not impossible than in a near future, temporary or definitive skin color modifications will be available, thus being some kind of new fashion. Maybe your aliens already have these kind of technolgies and uses just like we dye our hair/put a wig, put fake contact lens, or even change our clothing.

Another take on the "artificial color of skin", would be sickness/medecine. Some kind of secondary effects for a drug or a virus that changed some/all of the population skin color.

Or they did it themseleve, because their skin wasn't resistant enough (again, a side effect) to the sun/air/micro-organisms/etc. Again, as a tatoo, or directly in the skin, and evene maybe as some kind of advanced second-skin ? (if that ever counts as "skin color" ...)

Technically, I think that would be rather difficult to change the whole color of the body. However, if there is transhumanism, I guess there must be "transalienism" (or "transextraterrestrialism" ?).

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One reason for vibrant colour is toxicity. The poison dart frog is a family of frogs with a wonderful variety of colours and patterns; cyan, blue, black, yellow, orange, red. In comparison, frogs which are green and thus blend into their environment have weak or no toxicity. The evolutionary reasons for this have already been mentioned by other answers; in this case protection and reproduction. The more toxic the frog, the brighter its colours. This in turn means that lady frogs find gentlemen frogs with the brightest skin irresistibly handsome.

So you might have an incredibly toxic alien species which has a great variety of vivid skin colours and patterns, or indeed a species with a variety of toxicity and thus colour and brightness... though it might make interactions with them a little more awkward than usual. Hand shakes and hugs probably out of the question.

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    $\begingroup$ I think this shortens things a bit. Toxic animals can afford to be colorful (and as a result visible to potential mates) because potential enemies learned about the toxicity, whereas edible species need to hide from predators. $\endgroup$ – Burki Aug 25 '16 at 15:27
  • $\begingroup$ @Burki but with no references to poison dart frogs... what would be the point! :O $\endgroup$ – inappropriateCode Aug 25 '16 at 15:32

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