One of the more popular aspects of the Warhammer Tabletop when it comes to the miniatures is the ability to paint them whatever way you want. Most miniatures are painted using a 3-4 color palette. In Darktide players should be allowed to do the same thing. Skins on the store have 3 layers per part(under/upper/armor(acces.)
You can limit color choices by adding color palettes that are commonly used across the wh40k tabletop, and even theme/name them. This will help to avoid 100% neon pink cheetah print ogryns, but you should offer a wide range between colors available. Not 50 shades of grey.
If players can apply a color to each layer, free/store skins increase in value, maybe someone liked a skin, but did not like the color, if the option to change them existed that person might have used/purchased the skin.
Weapons can get a similar option as well, selling same skins in a different color should have never been a thing. Players should be able to customize weapons using a similar approach to the outfit/skins. This can be used as an excuse to just sell different models on the stores, not same models different colors.
Skins that have a camo layer, can have the option to change the camos color as well. This gives you an excuse to stop selling identical camo patterns in different colors, and instead sell different patterns. If you wanted to, you could also allow players to add said camo patterns to any skin they are wearing. And personally I would have never sold identical camos in different colors individually, it seems like a low blow, I’d sell one pattern that can be applied to all of your skins and one to all of your weapons, and let players customize the colors.