I'm not an expert on ivory.  Ancient ivory in walrus tusks seems to
be permanently discolored and stained, as seen in some old scrimshaw
pieces.  I had an (elephant) ivory keyboard in my shop which was
stained as yellow as can be; I felt that I would never get the ivories
whitened without severe damage through bleaching.  Much to my surprise,
however, just leaving them exposed to the ambient light was enough to
thoroughly remove the discoloration.  They're now white as new.

I've noticed that pyralin/celluloid keytops can also discolor, and I've
decided that this is due to the nearby finished case pieces gassing off
over long periods of time.   Often, the celluloid fronts of key levers
are discolored due to their close proximity to the keyslip, so it seems
reasonable that if the fallboard were to be kept closed for long periods
of time that it too would gas off in close proximity to the ivory or
pyralin and discolor it.

Sunlight is destructive, even indirectly; four-color posters left up
in display windows end up looking like they were composed in monochrome
blues.  Perhaps a similar effect is worked upon discolored ivory, that
the sunlight breaks down pigmented particles gassed off onto the nearby
surfaces.  This makes me ask the question as to whether mere ambient
sunlight works to "bleach" discolored celluloid as well.

Mark Ritzenhein