Black, blue, white (those Sapphire boards looked awesome), red, although I'd love to see a camo board, coupled with some of that OCZ camo'd RAM.
I have to go with black, but if you use a black PCB, then the other colors you use for it should match it somewhat well.
I take back my vote for silver and change it to gold. Black is good but not a very exotic colour to represent performance.
Black with blue connectors (now you see another reason I like the striker...) a copper colored PCB would make an interesting one, and the black/white camo I could see on an ASUS RoG board..
White would be pretty cool and you don't see it very often. However, I absolutely despise cases that aren't opaque, so I really couldn't care less what color my components are. Whatever works out to be cheapest (green) will be fine by me.
I said black, blue, and red. Why? I think that red and blue would be good for themed cases. Black is a nice neutral color. When you mod or sleeve the inside of a case, you want your features and components to stand out, not a flashy mobo color that might clash.
I actually quite like a nice vibrant green (a Forest green type colour say) but green in general does conjure up thoughts of every cheap board I've ever had/seen. Only ever had maybe one brown board ? made it look ancient even though I dont think it was. Red I think can be a bit overly garish and bright when it doesn't match the rest of the parts so I'd probably avoid that. Black, navy blue, silver, white and so on might look decent. Really I guess I'm just looking for something that looks fairly clean and professional and doesn't try to scream out ;p they are components after all.
I must say black, since it blends more in with all the other components and the paint.. Red is too visible for me.. xD
I don't really give a toss about the colour of a PCB, so long as the kit I bought does the job I bought it for, I'm not too worried. Shame you don't have an option in the poll for my point of view.