I am slow. Sorry. However, the skin on all of my Grot herd is now Catachan Green, the Crimson Fist marine is recognisably of that chapter, and there are other colours on some of the clothing of the Grots!
You might be able to see that one shoulder pad of the other marine is now brown. I am attempting to build up to yellow from a black undercoat, via brown. Eavy Metal did this once for Louen Louencoeur.