Std core board is not nearly as nasty as that of yesteryear. You can isolate the raw bevel by applying
some aluminium tape around the window (underside
) which will lift the board away a tad and provide
a barrier. The bevel will discolor, but not 'burn' the photo.
Daler 'Sand' Ingres std core is nice. Sort of aged Ivory.
I'm of the same opinion. White bevels look bad on antiquarian stuff.
Another way is to use a gold fillet, which looks classy but also should be isolated.