Warhammer 40K

Yea, it seams anything is ok; even if you are going for a theme. If you want a paint scheme, try looking outside of the game. Paint em like Australian Airliners (wild and varied colors)

As MadModelFactory pointed out, in warhammer 40k, it's a 'anything goes' policy. That's what's so great about this system. It really allows you to run wild with your imagination.
I painted mine (back when i still had them, not into warhammer that much anymore)
a metallic red (mixed blood red and mithril silver) with the shoulder pads and helments done in gloss black.

In WH40K basscially ANY scheme will be fine for the Marines.

There are 1000 Chapters and the majority are NOT documented, so go ahead design your own scheme and create your own chapter.

I used to have an army of Space Boars, one half of the figure was blue and the other purple, shoulder pads were opposite colours.

Had both a standard 40K as well as an Epic Army painted for the Chapter.