 Usually, the coating will turn black when heated, but coatings that turn blue or red are sometimes used.While an open heat source, such as a flame, can discolor the paper, a fingernail swiped quickly across the paper will also generate enough heat from friction to produce a mark.Multicolor thermal paper first became available in 1993 with the introduction of the Fuji Thermo-Autochrome (TA) system.