If you are going to cover it with enamel then I'd suggest sticking with enamel paints, or mixing your own.You don't really want to cover one medium with another as often they are going to cause problems down the road with cracking or adhesion.
Also if you are going to cover the paint with black enamel and view it through the back side then the transparency thing isn't as important since the paint will be sitting behind the collodion. You may want the paint to be more opaque to show off the color against the black.
Bookmarks