I think it's less about color and more about style. I've seen a lot more one-color tattoos that are blue, also, and I think that the less-intense color can make for pretty designs.
I tend to prefer line drawings to a lot of rendered art (both on people's bodies and in places like comic books) and there's less that could go wrong/look out of place in a one-color tattoo than a full-color tattoo. That's not to say there's no such thing as a gorgeous, tasteful multicolor tattoo. Someone I used to know had a pair of small, full-color peacock feathers on her abdomen, and I think that's some of the prettiest tattooing I've seen.