Excerpt from Mastering the Model Shoot by Frank Doorhof.
You might think that black is best, but nothing is farther from the truth. Working in a black room for a while is very depressing. However, working in a completely white studio is a disaster when you want low-key shots or good control over contrast. It’s best to use a studio with medium gray walls—a good solution against light spill, but not so dark that it feels depressing after a long week’s work. The ceiling color depends on the height. It can be white if it’s high enough, but it’s better to go for something darker. Regardless, most of the time, light reflects back easier from the walls than the ceiling when you take into account the angle at which you use your strobes (mostly aimed down).