The colors' effect can be appreciated when other things are kept equal.
Imagine a man, dressed in black and white, standing near Hepburn. Wouldn't they look better, especially in a photo or video, if her dress were not black?
I often notice that gentleman's and lady's figures show up as one whole piece during dancing or dance video, because both are black and indistinguishable. To avoid this, who shall wear a brighter color?
Mere contrast is a lot different from saying that a woman who wears black is masculine. Although your observation that men tend to wear black in smooth/standard, that doesn't mean that black never works on a woman (and not because she looks butch in it). Anyway, that's the concern for competitions, and the topic of the thread is women getting asked to dance, i.e. at a social.
I don't agree with this. Getting back to the topic of the thread (no, I didn't forget), I get asked to dance a lot more when I'm wearing a dress or blouse with coordinating skirt, and when I put some effort into accessories as well as hair and makeup. The more care I put into the ensemble, the better the result.