In addition to not being a particularly good miniature painter, I also have no fashion sense. Life has taught me one great truth about how to combine clothes, and that's that black goes with everything, especially more black. Left to my own devices, I'd probably just paint every miniature I have Black Legion and Guilliman Flesh. The problem with that is that it creates a really monotonous (er, monochromatic) army. Or maybe the result would be super cool and dark and moody, but functionally looking out at your tabletop and seeing a legion of black miniatures creates a bunch of mental cycles for you to figure out which blob of murky soldiers is which. Colour makes your army easy to distinguish, but if you dare take it a step further, colour also helps you identify individual troops.
If you paint miniatures, that might come across like a bold statement. My own feelings on it actually waivers, depending on several factors. Mostly, I agree [with myself] but here are the variables involved in whether I paint an army uniformly or with all the colours of the rainbow. Here are four of them.
From what I can tell in my completely unscientific research involving Wikipedia and Youtube, uniforms weren't really a thing until the 1700s or so (give or take a century). Prior to that, people were conscripted by their local lord, or else they were land owners who needed to defend their holding, and either way everybody showed up to the fight with the clothes and gear they happened to own. Even the professional soldiers of the Roman Imperial legions wore whatever clothing they could afford, aside from the armour issued to them by the state, (Again, I'm not a scholar, I could have details wrong, so don't quote me or take my word for it.)
Whether or not my research is accurate, the concept is persuasive enough for me to give myself permission to paint green tunics on my contubernium gladius (sword squad) and red tunics on my contubernium pilum (spear squad). My cavalry mattered less, because they're on horses. They're hard to confuse with infantry, which leads me to...
Colour coding a squad isn't too important when the squad is visually unique from other squads around it. My Roman infantry is distinct from my Roman cavalry, so it doesn't matter to me what colours the cavalry wears, at least until I get more cavalry and have a need to distinguish contubernia apart. In Warhammer 40,000, Skitarii Rangers and Skitarii Vanguard look very similar, from a distance. Their heads are different, so they're not identical, but I have to admit that Vallejo Xpress Velvet Red robes on one and Vallejo Xpress Plasma Red on the other might help them stand apart from one another. Vallejo Xpress Velvet Red on one and Vallejo Xpress Storm Blue on another would really differentiate them from one another, but dare I deviate from the box art quite that much?
And that leads me to...
I'm a fan of Warhammer lore, and that lore is reflected in the photos on the boxes of Citadel miniatures. Ultarmarines are blue, Dark Angels are green, and every. single. Adeptus Mechanicus is red. Because they're from Mars, and Mars is red.
I've seen some really cool ice-themed Adeptus Mechanicus online, and some green ones, and others. And they're fine.
But the Adeptus Mechanicus in the Warhammer+ shows and in the books wear red robes. It's iconic.
Silly as it may seem, I have a hard time deviating from red for Adeptus Mechanicus for the same reason I would never give Luke Skywalker a purple or red lightsaber. Some things just don't feel right.
Until they do, any way. I'm painting 1000 points of Adeptus Mechanicus, so at the very least, that Vallejo Xpress Martian Orange might eventually start looking pretty exciting.
Which leads me to...
Sometimes it's just fun to try new paint. This can lead to interesting results, some of which you like and some of which inspire you to find inventive solutions to a paint scheme that hasn't turned out like you'd imagined. I've rarely been disappointed by judicious experimentation.
Using colour can help you identify soldiers or squads in your army. More important than conforming to imaginary uniform regulations, it's refreshing to look at a squad and single out the medic in white, the strongarm red shirt, the scientist in blue, the commander in gold, and so on. It's a practical, but also it produces an army that's fun to look at.
Header photo by Seth Kenlon, Creative Commons cc0.