I bought some 8.5 X 11 waterslide decal sheets (like those for model cars & airplanes) for the color printer. <
http://www.beldecal.com> You print 'em off, then spray 'em with a coat of rattle can urethane to seal the ink. Then you cut 'em out, soak 'em and apply. After application and drying time, I used PPG Stratoclear over them. One of several drawbacks is that the colors are very translucent. The color of the paint shows through the ink on the decal. It seems that it only works well on a white surface.
So...I bought some more sheets that are white, not clear like the first batch I bought. Went through the same procedure and found that the white decals sheets are also translucent. I put one over a red tank and the white turned into a very pink color. Not good, unless you want pink on your paint job!
As long as you are putting the decal on a white surface it will work fine. Perhaps a black decal would work over most colors, too. Regardless, your possibilities are limited.
Last, the decals refused to lay flat on even a modestly rounded surface. I put a decal that was roughly 5 inches in diameter onto a motorcycle tank. Kept getting wrinkles no matter how much I cursed at it. /ubbthreads/images/graemlins/flame.gif
I haven't used them for any projects other than a few experiements, but I'm sure there will eventually be an instance where I can use them. Within the listed limitations I think they work fine.
Andy