My Dear Monsieur Pistolero -
I myself am struggling with this question, as I have accumated a tiny stash of various "faux ivory" materials with the intent of making my own grips, knife handles, belt buckles, bolos, etc.
It all comes down to what exactly the material is made of.
It all depends on what kind of resin or glue is used. I have found that Some Micartas may take stain, others will not.
I have some slabs of Corian, and some slabs of "faux Corian". Real Corian will stain, whilst some of the fakes will not.
If the material is a cast resin, it is almost certainly non-absorbant and thus impervious to stain, but it may discolor with heat or UV.
The only thing I have found is to try different things on a small area that won't show. Sometimes a stain will only penetrate after soaking for days ; Take notes and label things well or you may end up like me, with one perfect example, but I don't know how to reproduce it :-( ....
Also, be careful trying heat or UV, as it could destroy the grip :-(
One thing that seemed to work is to gently buff on a power felt wheel with red or brown jewers rouge. It seems to discolor some materials with heat as well as the rouge putting a light brownish color - but that is more like red stag than aged ivory.
good luck
yhs
prof marvel