Reading about thermochromic pigments, I found out there is a common type that goes from transparent to black in a relatively short temperature interval. I decided to try several mixtures of this with a white enamel, to produce a pigment mixture that turns white with heat.
After some successful trials I designed a 3D-printable 7-segment clock. A total of 28 custom cut segments be needed for the whole project.
This was one of the first tests with the paint, limited to half watt. Later experiments yielded much better segments.