I was wondering how easy it is to paint a translucent plastic and keep it see through. Is it even possible?  I don't want to destroy a perfectly good light pipe, for instance.

Tamiya makes translucent paint specially for that.

^ yep.

 

Most brands of model kit paints have clear paints in their assortment. You could also try glass paint.

 

Trouble is, you're limited by the color of the plastic. Clear transparant plastic can be painted with no problems, but if the plastic is already a color, then the end color will be different from what you intended: painting clear yellow over clear red, won't give you the clear yellow you painted, but some kind of weid mix, because the red is still there.

 

EDIT: btw, some brands (like Humbrol) even carry clear primer (Humbrol Base Cote)

I have made translucent pieces different colors with Testors Racing Finish translucent colors.  Here is an example:

 

 

 

 

 

What I did was soak the Shockblast parts (which were yellow) in some nail polish remover for a bit.  I was actually able to remove the yellow and then paint red on top.

 

Here's another shot:

 

 

 

 

I just got through with a Grimlock sword on my latest custom (of which I'll have a gallery up soon) using the same method.  Hope that helps!