Laser etching is a great process but I've often wished I could have higher contrast etched text, colors, etc. Then I realized I could do that very easily. If you're laser etching acrylic sheet it already comes with a protective paper covering which makes an awesome paint mask.
So the basic process is deeply laser etch the panel with the protective paper in place. Paint though this protective paper mask. You can be as sloppy as you like. Then simply peal the paper away. Now you have super clean highly detailed etchings, and the paint is protected because it's actually recessed into the plastic. (That's why you use a deep etch.) You can use any color or combination of colors you like.
Step 1: Deeply Etch the Design With a Surface Mask in Place.
Do a deep laser etch of your design on acrylic with the protective paper in place. I suppose you could also try this by pre-masking non acrylic object. Like putting contact paper on wood, etc. But I haven't tried it and I'm not certain we're allowed to vaporize contact paper in the laser I have access to.
Step 2: Paint the Parts of the Design
Now you can paint the various parts of the design. I use standard art acrylic paint. High contrast uniform colors work the best. I've tried metallic colors as well, but those are more sensitive to stroke direction, evenness, etc. So they won't look as professional. (Although I've had some nice effects using that technique.) You don't have to paint carefully, just make sure you get the colors down into all the nooks and crannies.
You can use any color you like, and paint different areas different colors, etc. Here you can see me making a set of gear dials with different colored accent lines. I'm sorry this doesn't match the previous image of what I was laser etching. I wasn't originally trying to document this process, so I didn't take a picture of every step. Jim said I should make an instruactable about it though, so I am, minus the original laser etching/cutting image.
Step 3: Peal Away the Paper Mask.
Then you simply peal away the protective paper. I usually wait until the paint has mostly dried, but with acrylic paint you don't have to wait super long. I sometimes use the head of the plastic toothpick on my Swiss army knife to unmask very detailed areas. It doesn't scratch the acrylic and you can just scrape around until all the little masking bits are gone. Or just use your fingernail.
The nice thing is that the because the final painted areas are recessed they are super durable and can't be rubbed/scratched off easily.
If you're laser etching a smooth surface that doesn't have protective paper, you can use paint, and a rubber squeegee to get the paint down into the etchings, but that's more of a pain and you have to do a bit more clean up. This system with the acrylic is ridiculously easy.
I haven't tried this with spray paint, or airbrush, but it seems like it aught to work just fine.
Step 4: Enjoy the Fruits of Your Labors
Here's a final close up shot of one of the gears. You can see the amazing detail in the skull's eyes, etc. The whole gear is only maybe an inch and a half across. In this photo you can see I still need to scrape a bit of the paint away from the side of the gear, and from the German Cross.
Well, have fun with this. I know it's opened up a lot of possibilities for me.