David,
Do you have any other colors of paint? I wonder if anything strange would happen with an orange or red. I would suspect it wouldn’t. As you’ve mentioned, the chemical in the white paint is what “burns” the glass/tile and makes it black. I’m suspecting that the black paint creating the white emboss is actually b/c of small fractures in the glass as the laser heats up the black paint. I could be totally wrong though. I just wonder if the only options are black and white.
This does bring up an interesting idea on glass. Can you paint it white and laser etch it to get the black. Then, clean it off and paint it black and laser etch an inverted image to make the white and get a more dramatic effect? I’d think this would only be helpful on glass as you’ve already got white when doing the process on white tile.
Just an idea.