You could also try dyeing the tubes of the gauss weapons. Keep the transparency but change the shade. Here is something I found after a quick search:
http://www.pburch.net/dyeing/acrylic.shtml
More could probably be found with a more thorough search.
Or, if you don't feel like playing chemist, you can buy translucent acrylic tubes in the appropriate size for the weapons and cut to fit the length. That may be a better route to go.
Hope this helps. And post pics for whatever method you choose (dyeing, painting, whatever).
Cheers
Automatically Appended Next Post: NecronLord3 has a good suggestion.
But if you are doing a large number of them, cutting a rod of acrylic may be more cost effective.