Cold thermal stones don't display the correct color since the Troubled Waters update. When placed in a fridge or by an endothermic fire for a long time they appear white instead of the frozen blue look. And sometimes when only half frozen (light blue color normally) they appear yellow as if they are warm instead of light blue.
Edit: This happens in every season except spring.
Not sure how to reproduce but this use to happen in 2019 as well and now it's happening again after the latest update.