50-90'C color changing chemical to use as warning marker for hot electronics? I am testing electronics with many potentially unstable contacts, and I wish to monitor if they rise above 50 to 90 degrees Celsius, i.e. using carbon paper that would darken progressively from 50 to 90.

The substance should stay a specific warning color if it goes too high and not change hues again same as thermochromic pigment, so that I can detect high temps.

I hope you know a chemistry trick that could help? Thank you.
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.