When you etch a tile in the first place, you put a resist material down onto the tile. This material resists the impact of the abrasive on the tile. Either before or after you apply the resist to the tile, you produce a design on the resist, which allows you to blast exposed parts of the design while protecting the tile in other areas.
After blasting to your desired depth, simply blow the dust off the tile (with an air nozzle) and spray paint, leaving the blasting resist on the tile as a paint resist. You can even use different colors by covering one part of the design while spraying another part, then covering the first color while spraying a second, and so forth.
-Ruth Dobbins, EtchMaster