1991 Chevy Camaro The engine coolant

Engine Cooling problem
1991 Chevy Camaro V8 Two Wheel Drive Automatic

Alright, I was driving my camaro yesterday, and i had it in drive when i probably should have been using the automatic overdrive. So when i got on the highway, the engine started to get really hot, so when i got it home i opened the hood and i saw some brown liquid coming out of the coolant resevoir bubbling out of the top. My friend came over to look at it and he said the thermostat was broken and i needed a new one. I checked it the next day, and the liquid had turned kind of a greenish color, but it was still full to the brim. My friend then said not to turn it on or else it might ruin the engine. Im just not sure if i should get a new thermostat, or have someone look at it, or if everything is alright, or if it's safe to drive the car. Thank You.