Black seems to work the best for me but other colors are effective also. I carry black,brown,olive and black,olive,white and purple. I usually tie them with a conehead and crystal flash tied into the tail.
I fish olive most of the time. I tie almost all of my buggers with rabbit strip tails instead of marabou. This probably isn't technically a true wooly bugger anymore but it is a variation that I find to be very effective. I just like the action of the bunny strips a bit more than the marabou.