How to paint letters cut in porous stone?

[VisualThinker7@zzzzzzz]

I would use masking tape on the flat surface of the stone, and use a primer
before painting, just as you would if you were painting porous wood, or metal.

Not only is there the question of the paint soaking into the porous stone
unevenly,  and giving you an uneven finish, there is also the possibility  that
minerals from the stone could be pulled out by the liquid paint, mix with  it,
and stain the color in places.