This is a very simple trick, but it will give your robots a very nice look if you use epoxy on them. Espcially if you coat one of your bots in it to make it dust/water proof. I just added some 3d images of the tutorial, so you should have no trouble following it.
1. Get some food colouring.
2. Mix the epoxy for a couple seconds.
3. Put 1 drop of desired food colouring color on the epoxy.
4. Continue to mix for a little longer then what the instructions say.
5. Use epoxy as normal and when it dries, the color will be very nice.