One of the toughest substrates to paint. If you simply apply a latex paint, there's a possibility that the water in the paint will cause stains to leach out from the tiles into the paint film leaving a yellowed, ugly mess of a job. To be safe, it's best to prime them with an oil-based stain blocking primer. Allow that to dry for 24 hours then apply two topcoats of satin or flat latex whichever you prefer. Personally I like SuperPaint latex satin from SW. Be sure to get primer into the seams as that's where a lot of the leaching can occur.