Since most will agree that fish are oportunistic feeders, i subscribe to the belief that fish see what they want to see when it is dangling off their nose. I know that if they are eating #18 olive emergers, they probably wont eat a #10 royal coachman dry (but they might

), however, i think that color, as long as relatively close, is less important that size and presentation. This is one reason that i really like biot bodies on nymphs and soft hackles. I think that an olive biot, will actually cover several portions of the olive/green spectrum, and thus make it easier for a fish to see what htey want to see.
I use biots in green, olive, black, brown, tan and grey, my most successful color being grey. Because there is an aspect of translucence on the thin side of the biot, and darker color on the "rib" side, i think that it most closely illustrates the contrast and nuance that is being debated here.
Just my thoughts.
Related directly to Olives, i think that a dusty or greyish olive is the most successful. Thats just from personal experience in Maine and NH.