Mechanical Keyboards

So this is a variation on dye sublimation, one of the main ways legends are printed onto single-color keycaps. The process is basically your second idea and exactly as @Uranium_Green says.

• Coat the area in ink from a marker meant for permanently decorating cloth or various polyesterish plastics. The intended heat source is a glorified clothes iron made for DIY crafting.
• Feed the Laser a simple bitmap image and run it low and slow, so it heats up the plastic but doesn't melt or char it like on several of the earlier trials in that photo.

This should cause the heat-activated ink to integrate more or less permanently to the polyester-based PBT plastic. Then I clean off the unlasered ink with isopropyl alcohol and/or acetone. The zapped ink doesn't fade at all, though as you can see it's not super vibrant.

The sweet spot to do this without charring or melting the keycap is pretty small, though though there's probably another configuration where you use higher power and quicker movement. There might also be one at even lower power and longer time. I'd seen several people try it, but with weird projects on cheap lasers you've typically got to find your own settings. Also, don't laser ABS; very bad idea.