I know this thread is several months old. But I was searching for a way to do color lithopanes. Painting is too much work...I guess I am lazy... I did find another solution for smaller corian projects.
Reference:
http://www.vectric.com/forum/viewtopic.php?p=18356
1. converted the color image into a CMYK image then deleted the "K" channel which is the black or shades of gray that the lithophane already represents.
Then glue the transparency to the back of the lithophane being sure to line it up just right.
NOTE: They look the same as a regular Lithophane from the front without back lighting (and if you give them a light spray of mat-white lacquer you can't see the transparency from the back side also).
2. I used 3M Super77 spray adhesive, it has no affect on the ink and gives you a little bit of time to line up the print before setting up.
This adhesive can also be used to attach a lithophane to glass...
I think I want to try this and see how it works out