I’ve been studying the Polaroid Z-Ink paper recently. This paper has built in dye crystals that change color when heated. By carefully controlling the process, pictures are printed without the need for an ink cartridge.
More information can be found here:
Artists also use Z-Ink paper in creative ways by using hot wires or other heating devices and "paint" on the paper, as in this example.
I printed a picture and peeled the layers off to see where the dye crystals reside. Surprisingly (to me), I found that there were 2 layers. The top layer has magenta and yellow. This layer is a plastic coating which is really flexible and kind of like a decal. Behind that, there is a cyan layer on a more rigid thin plastic film.
Here’s a photo of the 2 layers, peeled apart.
I also found that if you heat/cool the paper in spots, it will change colors back and forth between a couple of states, usually white when hot, and back to cyan when cooled. You might be able to make a 7 segment display or something like that by heating/cooling segments, but I don’t think it would work very well. I had also hoped that all of the dye would have been on the decal part, so I could apply it to glass.