i'll have to try some red markers... would think that might be hard to get an even coat.
looked around for different colored bulbs of that style but didnt see any, didnt think to try an aquarium store... would be more expensive probably.
isnt a gel usually like a piece of plastic? the way I have the lights setup is exposed so I dont think a gel would work
and I will experiment with spray paint, maybe find like a transparent varnish
the light I made is similar to this
http://www.thetubelamp.com/
just with 4 bulbs and not quite as fancy a box.