Thermal stone state colors (and thermal stones in general) should work off absolute temperature, not ambient temperature.

[Uncrushed]

The whole thermal stone situation is very confusing.  If the stone goes from zero to 70 degrees (character freezing to character overheating), the icy color (not shown here) should show at 5 deg and the hottest color at 65 (or 55 or 5 deg less than however hot it can get).  I've been using the Insight mod to inspect my thermal stones and absolute temperature is a much more sensible system.  I used to run out of base when the stone reached red hot or ice cold based on the color, but it was often only halfway heated or cooled.