Really a waste of time with polymere it will not take any stain. The polymere grips that have color have it cast in.
Not always, it's worth trying/experimenting on an inside surface to see if there is an affect.
My work is in plastics, (26yrs/current ) just trust me, with the myriad of "polymers" used in our current market, it's worth trying.
I do know that coffee or tea will stain ivory and bone,, but won't last. (been there-done that).
The key to "aging" is to lend a patina of color to the edges of and too minor imperfections found with common wear.
Many times that requires applying a stain or darkening agent to the entire item,, then polishing back to bright the areas of friction/use.
In the case of plastic polymers, sometimes it isn't about finding a stain that will take to the surface. It's about learning how to polish the finished surface again to it's natural/new state leaving the edges,, after the entire thing was coated.
Get it?
Just a hint,

That "coat all and bring it back(?)" is a common technique used to "age" a brand new hand carved curly maple $3,000 rifle, and/or most furniture found at antique shops,,