Get a few scraps to practice on. You may have to seal the stone, as some stones are porous or have streaks of other material in them that may mess up the ink.
Optically, you want to be opposite, to be readable. So a bright/light stone = dark ink, dark stone = light/bright ink. Give them the advice that a different color combo might be better. I would show them a sample of the gold on pink, because sometimes people have an idea in their head that just will NOT work, and until they see it in front of them, they won't get it. And make a few other color combos, as alternate ideas. But if the client wants something, they are paying for it, so you give it to them.