Put it in tea or coffee (both only room temp).
A table spoon of either from the morning brew left to cool, then drop the bead into it and let it set for a while.
Some brands work better than others!
The ivory will take alcohol based (wood) stain and darken nicely.
You can do this after it's installed if you want.
Those colored felt tip markers will color them too. If there's a set around with the different colors, the yellow or brown work well.
Easy to do BUT easy to over do!
Sometimes works better if you damped a small cloth patch with alcohol or acetone (which ever will disolve the marker substance),,swipe the marker(s) on the patch,,then rub the bead with the patch. Avoids getting too much color on the bead.
Water based artists colors penetrate just enough to color the ivory to a nice look.
Used to put a couple of used cigarette filters in a small jar with some alcohol. Break up the filters, let the alcohol & nicotine mix and use that to color ivory. That really works nice. Cap it and re-use!
But I don't smoke cigs anymore.
..not a bad yellow base wood stain either
Lots of things can be used.
Stay away from using anything that is high temp.
Ivory is very dense but still porous,,so it will absorb color.
Heat will damage it and a tiny sight bead may fracture.
Grips and grip caps, ect can get cracks from a dunking in hot tea or coffee. ,,and they don't look like mature age cracks of vintage ivory either..