Thermochromic (Heat Activated) Pigment. As the pigment is heated, the pigment becomes color-less; alternatively, when it is cooled, the color is returned. You can mix this pigment with a wide range of mediums including acrylic paint, fabric dye, nail polish, plastic, ink, and more!
https://www.tkechemical.com/pigment/thermochromic-pigment/thermochromic-pigment-rainbow-pack.html