I used a laser cutter at TechShop, a phenomenal resource for artists, builders and craftspeople of all kinds. They have workshops in several US locations with tools and space for woodwork, metalwork, industrial sewing, electronics, arduino, and more.
Step 1: Engrave artwork from a digital file. I did mine backwards on the back of the glass so it would show through right way round from the front and the color would glow through.
Step 2: Apply paint or marker.
Step 3: Scrape off excess paint or marker.
Click here to see a short clip of the laser in action.