The other weapons could be made golden with a cheat device. The message that describes it says that it is a paradox, and that this weapon doesn't exist in the database. Their logo would be similar to that of the Morph-O-Ray, but the background would be red instead and have a no sign on it. There is no improvement to them, other than that.
In Going Commando upgraded weapons are sometimes named gold weapons, probably because weapons' icons are gold.