Find some white pigment at your resin/fiberglass supply house, and then mix up some West Sys. laminating resin with the white pigment in it.
That would work fine.
The nice thing about Marine Tex is it sets up quickly and is really easy to file back/blend in with adjacent undamaged surfaces. Put a little paint or some of the pigmented West Sys over the Marine Tex and you'll have a really nice looking repair.
Are you "prepping" the areas that you've painted?
Take the time to blend in the scratch with some 180 grit abrasive paper, then wipe the area clean (quickly) with a tack rag with a little acetone in it. Your paint should stick just fine.
Often the problems with painting any sort of plastic have more to do with the mold release that remains after manufacture. Abrasive blending and a light wash with acetone should give you pretty much virgin plastic.
Hope this helps,