What is the best color background . . .

. . . for head shot portraits? Does it matter?

I like a neutral light mottled gray...

I like a neutral light mottled gray as a BG color because, used alone it is very flattering to most complexions, hair colors and wardrobes.

Additionally, it can be modified very easily by placing a colored gel over your background light.

At almost a tie for my favorite BG color is a non-reflecting black such as black velvet.