As many of you know, surgeons use green coats because that color is complementary to red, eliminating problems of double vision and ghosting effect: "green may be especially well-suited to help doctors see better in the operating room Because it is the opposite of red on the color wheel." http://scienceline.org/2008/09/ask-locke-greenbluescrubs/
For us mortals, busy all day looking at the white pages of books and notepads, which may be the right color? I always had white and beige keyboards, gray at best.
A black keyboard, or colored, can be annoying? Can it stress your eyes?
Most people don't spend much time staring at the board, especially this who use mechanical keyboards, so I don't think it really matters too much. High contrast legends can be more of a factor than overall board color, IMHO.
This is coming from someone who changes their keycaps regularly and is always buying new ones
Penumbra (with Solarized alphas) has remained the longest on a board out of all my sets, if that says anything. It's designed to be less straining on the eyes, but I just love the colors and feel of the set, I can't say that eye strain has been a factor in that.