Crescent 8P rag mat (100% cotton) in virgin white, off-white or bone white (the differences between virgin white and bone white are subtle but detectable in appropriate lighting), never anything in the grey or black spectrum. This is all I have ever used for any and all prints produced over almost 40 years now. The rule I have for any print, colour or black and white, is to let the print speak for itself and never be upstaged nor overpowered by coloured mats or a "shouting" frame. I have been to a few exhibitions where beautiful photographic prints — colour and monochrome, have been patently screwed by untamed, outlandishly overbearing mat and frame jobs.