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The sciency psychological answer is red... it's like a weird, visual aphrodisiac. I'm not saying that's the color I find the most beautiful on a woman, but then I did clarify that as the sciency psychological answer.
Besides that I'd have to base it on hair color. For black hair, I'd go with a deep blue, with brown hair, I'd go for a light forest green, and for blonde I'd go for a shade of black or gray, with some other accent color. (My sister has a black dress with a red belt that goes with is, so something like that). For red hair, I'm sorry to say I haven't met enough red heads to get a good idea of what does and doesn't look good on them.