For my silver gelatin prints, I have gone with a bright white -- usually a silver metal frame (Nielson profile #11). For my warmer platinum prints and carbon prints, I go with natural white and black wood frames. Usually 8 ply for the natural white.

I find it is a rare B&W print that can work well with a black mat. I used gray for a short while many years ago and quickly tired of it...but it was not as "clean" looking as some of the nicer grays now.