Confused on resistance colors of enemies/bosses? Watch the Hypothermia skill video lol

This was never explained for a newbie like me lol If you watch the video for the skill "Hyperthermia"... it actually states/shows it lol Just found this out after getting close to endgame/maps.

Figured Id post this, since one thread I came across on Reddit, they were still debating on the red/grey meaning.