As Marks says Resin-s for white is a given. And he's right, nothing will age the color past being bright white. Any generic toothpaste will work to clean up ivory, if it can be cleaned. I have used "Crest Complete" and the A&H baking soda version, both to good effect.
I own may be 3 dozen pair of ivory? Have owned double that at one time or another. The oldest that I have right now might be from 1904. Likely not that old though. It has some color and grain but still mostly white. A couple other pair are similar. Only one actually going "yellow" in all my ivory. And they were only fitted in the last couple of years. One pair came from the same mammoth tusk as Tom's yellowing pair above. Still white as a baby's ass pretty much, except for slight mineral staining. But they haven't changed in color from the blanks Tom cut to now, 2 or 3 years later. That said most of my guns/grips have been out of the light and in gun rugs since I've owned them. And they don't get handled a lot these days.
I have stained more white ivory to make it yellow/brown to looked aged than I can count. And a majority of the time, then ended up cleaning it with tooth paste to get it back to color I find acceptable again or just to get it to high light a carving. Ivory is a tough material and fun to work with. Don't be afraid of trying something with ivory. As long as you aren't cutting it.
Pictured from top to bottom:
Colt with my "yellowing" ivory
Uberti with Resin-S imitation
Colt with Tom's mammoth ivory (bright white!)
1904 Colt with old ivory (but likely not ivory from 1904)
This is ivory that was white.
stained now with tea to highlight the carving by Dennis Holland.