I've seen several people do something very similar to their notchie tail lights. I even did something like that once (although with larger circles, for more light output).
It's really easy to do. You just sand off the black lines from the colored lenses (wetsanding with 220 grit works), then cut some circular pieces of masking tape. Stick the tape circles inside the clear lens covers, and paint over them. After the paint is dry to the touch, pull out the tape and put the tail light assemblies back together. The tricky part is getting the tail lights apart without breaking the plastic tabs that hold them together.
Back in the day (we're talking mid-1990s), I also modded a set of notchie tail lights so they had two thin red strips going all the way across.
[This message has been edited by Blacktree (edited 06-02-2010).]