Take a look at Crescent "Bakers White"
Their "Select" (Alpha Cellulose fiber) #9800, or their Black Core #9900.
It's a very neutral bone white that I find works beautifully with any Selenium toned B&W image.
It is equally comfortable with Brown or Sepia toned prints.
A truly universal color for monochrome photography (and ideal for color, too).
Over the past 25+ years of fighting with the fickle foibles of the framing industry and their penchant for pushing the "Latest Trend" in designer colors, (determined by designer artistes pirouetting around in their tu-tu's), I finally found the softly subtle clean Bakers White to fit my presentation style beautifully.
If you're comfortable with something less than Rag or Alpha Cellulose fibers, Bakers White is also available in their International Whitecore #SRM3400.
Phone Crescent at 800-323-1055 and ask for their matboard specifiers.