50-90'C color changing chemical to use as warning marker for hot electronics?
There are commercial  sources for thermochromic permanent-change ink, but you might also try thermal printer paper. You might need to check if there is a time-dependence on the color change, as well as temperature dependence. The printer paper is available at Amazon, Staples and elsewhere for less than US$20 / package, but you might just test a cash-register receipt, many of which are on thermal paper. According to this PDF, most start to darken at 79 C.