Define 'practical colours'.
If the aim of warfare is simply to kill as many people as possible then your colours are, practically speaking, very limited.
If, however, your aim is to impress, intimidate or let the civilian populace know that 'hey, the good guys are here now!', then it makes more sense to drape a big red-white-and-blue banner over your shoulders and throw piñatas to the populace.
Which leads to my point: If the state of military action in your world is such that being able to easily identify troops (either so your enemy knows who is hitting them, so you don't shoot your own people or so your opponents think 'golly, what a sharply turned out regiment!' and flee) is a higher priority in the minds of the people calling the shots then you will end up with brightly coloured, brashly nationalistic uniforms.
These may be thoroughly impractical for the purposes of not getting shot, but brilliant for whatever purpose the shot callers want.
As an example scenario: A government think tank in the Federation of Notreal decides that civilian resistance will decrease if soldiers appear nonthreatening. Notreally Soldiers in their rival Baddudistan are all issued mandatory 'happy face' body armour. Now you can easily tell the Notreally soldiers from the Baddudes because one side have happy faces on their chests. The people with boots on the ground know this is ridiculous, but it doesn't change their orders.
As it turns out the smily faces reduce civilian resistance, Notreal annexes Baddudistan and the idea of 'smily faces are better than camo' becomes firmly entrenched in the national psyche, remaining part of the uniform even when it's a bloody stupid idea.
Now the above is an absurd example, but it's meant to illustrate that a different uniform might have an advantage that isn't immediately obvious to the people being shot at on the ground (or actually justified), but which then becomes part of a national identity that people don't want to sacrifice (which may be important in an all-out conscription led war of attrition).
And the infantrymen will hate their officers forevermore.