Thermochromic pigment that changes at around 50 °C / 120 °F?
The easiest way to obtain such a pigment is to mix a blue and red thermochromic pigment together, one that changes at 32 and another that changes at 50 dregrees. They aren't very easy to find, for the moment i have found the 32 degrees pigments on ebay, and a 45 degree pigment on aliexpress: