You can get clear primer for water PVC. If the inspector trusts you he will let you use it. However if the inspector has any doubts about your abilities he will demand you use the purple stuff (including rip out all your work that is probably perfectly good just so e can see you used the primer). The purple is just a dye, it serves no purpose other than to inform the inspector that you used it.

The reason you don't need primer on electrical is the connection is only required to be mechanically strong - water leaks are just fine. If there is any possibility of water getting into pipe you are required to use wires that have a waterproof insulation.