I remember a little solar paper kit I had as a kid, it came with these square pieces of what felt like origami paper. The paper was in a black plastic protective pack, and was blue in color. When you put it outside, you'd put leaves on top of it, and weigh it down with a clear piece of plexiglass and set it out int he sun. Now, I don't remember how, but I do remember somehow that resulted in a blue outline with white shapes. It's been so long, I was probably toddler aged or something.
I think that would be your best bet, though, is finding out what dye they used and incorporating it into the sand/cement mixture that you use, and then lasering it so that it changes color and you have a "stencil" printed onto the work to work with.