I had some work done on the AC system and the shop said they had to drain the coolant to gain access. Today I noticed the car was overheating at idle, when I checked the coolant it was yellow/green. It's my understanding that these take only the blue coolant, is this correct? I called the shop and they said to just top it off with distilled water when it's cool. This all sounds very suspicious to me, what do you guy say?