Testors Decal paper

I've used both the clear and white sheets. While the white is not opaque enough for me, they work quite well. You will need the sealer that they sell after you print your design on the sheet, or the image will wash off in the water. If you print on the white, you will have to trim the design close, or you'll end up with a white border.

I prefer other papers, as the Testors seems to be more expensive. I generally use the stuff sold by Micro Mark.

Making dark decals to go over light colors is easy.  Doing light colored decals to go over dark colors is hard.  Inkjet printers do not print white, and they print very light colors by printing low opacity.  So, for light colored decals you need to use white paper.  But the paper is white everywhere, and this leads to a dilemma.

You either need to trim the decals perfectly with a hobby knife, or else make your background color match the paint exactly.  Even if you do that, you will see a slight white outline from the decals afterward from the edge of the white layer.  This has to be carefully colored somehow after application.