Firstly the girl needs to feel confident in it. Why? Because confidence is REALLY sexy, plus it often makes her more assertive and daring which is also sexy. Secondly the type needs to fit her body formes. This one is a bit tricky, and i guess you girls probably know better there (We guys just know it when we see it). If not, ask people working on victoria's secret, or something? I duno. As for color, i tend to generally prefer warmer colors, but that's really individual. Try to find out what your guy likes :)
Thing is it TRULY depends from guy to guy. Most guys like the sexy look: Black lace and such. I'm not saying I don't like it I love it... BUT I prefer a girl in normal pj's. It's so cute and for me personally when cute gets naughty... omg people aren't going to be walking properly in the morning =P.