Black, white and gray. Great for fashion images where the clothing is the object of the image and the clothing gets the brunt of the attention, but for portraiture where the subject is the object of the image these are boring, colorless and boring, and really boring. Boring backgrounds assist in making boring images. Yes some people look good on black, some look good on white, and some look good on gray. Nearly everyone looks good on blues, greens and browns, especially if they are not seamless all one color but instead are mottled, or swirled with a variety of several colors like one gets with a muslin or painted canvas background.
Of course this is only my opinion and I'm not a real pro like some here.
Benji