I keep "The Amateur Photographer's Handbook" (A. Sussman) by my beside, and last night, I discovered that there are such things as viewing filters, used by B&W photographers, to preview the monochrome scene, it seems.
Did I understand this right? Are they in use at all?
I never heard of them before, but I now see a few on online vendor sites.
You can a similar idea of how an image will look in monochrome (albeit not black and white) by wiping though almost any medium color filter such as amber, yellow or orange.
The classic viewing filter for black and white photography is the Wratten #90, a dark amber shade. In use it is brought up to the eye for a quick glance rather than a long stare. The eye/brain complex adapts quickly and tends to cancel perception of the filter effect.
Blue filters were used as viewing filters in the days of blue sensitive film. A cyan filter would work for orthochromatic film, I imagine, though cyan filters aren't very common outside a Rosco theatrical filter sampler pack and not even then. A very deep blue/purple filter would work as a viewing filter for wet plate & tin-type.