I painted marines in orange and blue long ago...that was an interesting time. I'd consider going back to it again if I found an orange paint that I liked. And, for some reason, had a need to do a new Marine army.
My Chaos renegades are in a split black/white scheme. I love how they look...but painting them is tedious and slow. With all the trim on Chaos models, I effectively end up painting each model three times. Not that much fun. Worth it for the look...but not my favorite.
I'm going to have to side with Steg and say Sallies. The green/black looks good, flames can be improvised all over the place, and the fluff is a nice change of pace from the rest of my 40k armies. I mean, they're still psycho-indoctrinated killing machines. But they're psycho-indoctrinated killing machines who care, and that's what counts.