Chevy Impala 2010 Engine Hot Turn Off A/C Warning

This has happened a few times but it was more alarming this instance
because my car had been parked outside, covered in snow for 3 days prior.

I had warmed it up 2 hours earlier, took it for a drive then 4 mins later I got a
"Engine Hot Turn A/C OFF". I came back parked it and 3 days later when I
turned the car on, same warning so I again turned it off. Came back 4 days
later and same warning.

The manual says this message displays when the engine coolant becomes
hotter than normal ... but how can that be when its been parked outside in
20-30 degree weather for days?

This happened in Oct  so I took it to a shop and they said the coolant levels
was fine. Now in my opinion the coolant looks very dark ... do I need a
coolant flush or is it something else like a sensor? BTW the Temp gauge is
still on C(Cold). I attached a picture