I used HP color transparency film.
1st I converted the full color image into a CMYK image and then deleted the "K" channel which is the black or shades of gray that the lithophane already represents.
Then I glued the transparency to the back of the lithophane being sure to line it up just right. I used 3M's 77 spray adhesive.
I then gave it a light coat of mat white lacquer paint over the transparency so that the color "magically" disappears when the light is turned off...