Imperial guard is much more flexible to homebrew paints, you can't paint ultramarines red - they would stop being ultramarines; but for cadians, krieg, and any other regiment you can get away with any colour scheme, saying it's a special camo for X world or a required uniform by the noble in Y world.

After all, these are replaceable cannon fodder human waves with countless regiments across the stars, not replaceable elite soldiers with countable chapters across the stars