I had meant to update a month ago but I came across a new way to do the color matching that I liked so much that I rewrote a huge chunk of code to implement it. Pretty happy with the results.
Here is the new interface that allows you to select the size of the finished lithophane and other items.
Change log
Dramatically faster
Can choose lithophane resolution
Can choose extrusion width
Can choose lithophane size
As always feedback, good or critiques, is welcome.