I personally don't think color makes too much difference. At least on my waters, there is never really a blanket hatch of any one species. Rather there are a mixture of several different species at any particular time.
Also I fish fairly fast waters, and I'm actively moving my fly as well, so the fish never really get a good long look at it. I personal want my fly to act as something alive, rather than only relying on the fly to fool the fish by looking exactly like the bug.
With that said, I fish mostly light olive or tan colored bugs. However when the mood strikes, I'll fish a fairly bright yellow which seems to work just as well as all the other colors.
I most fish a size 14 fly, sometimes going up to to size 12, very rarely going down to a size 16.