Bright yellow for me! It's part of the reason for choosing Fists in the first place, which too much dirt and battle damage detracts from. The opposite I suppose would be taking a dark scheme like Black Legion, Raven Guard or Iron Hands then giving them all pink fluffy legwarmers. Doesn't seem right does it...?
For the aquila, I don't think I've seen the exact colour covered anywhere but I know it's not company-specific - so fluff-wise I want to say red. Trouble is, it would look better as black if you're using black trims already.