There used to be a heavy, lead based paint in white and black but of course lead is now evil. We use industry Litho sprays such as grey shadow and black shadow.
In the case of hand cut (chisel) letters on marble, which is porous and soft (modern stones are much harder granite) they’d have to be hand painted with a pointed brush using a flat black that is composed like “Krylon Indoor/Outdoor Flat Black”. This fast drying paint if not applied too heavily per coat won’t tend to soak into the stone causing a fat fuzzy look around the letters. The job on the stone in the thread took some caring person a decent amount of time to do.
Is the supply of good gray Barre granite (Rock of Ages) holding up? I know they were having trouble a while back.