Dull the tiles by running over them with a belt sander and extra-rough aluminum oxide sandpaper. The glaze surface won't come off, but you should be able to get it free of shine.
Wipe up the dust with a rag.
Cover the whole tile surface (including the grout lines) with sealing primer, using a paintbrush, and going all in one direction. Keep the primer smooth, thin and flat. Let it set overnight.
Paint the tiles in the same fashion as you laid the primer, in a thin, even coat. Don't worry if you can see the primer through it; the important thing is to avoid ridges or drips. Let the paint dry for eight hours.
Apply a second coat with the same procedure, then a third. Let the third coat set for 24 hours.