1999 Buick Lesabre coolant got dark fast

Coolant was pink so I guess it was Dexcool. Over two weeks it gradually became dark red and looked like it had dark dirty oil in it. Smelled like oil I thought. I think my mechanic topped the overflow with a little bit(maybe half cup) of antifreeze that was not pink. But we did pour in a small bottle of leak stop into the antifreeze. There was a very slow coolant leak. I think he said the intake manifold gasket was on its way out. That would leak antifreeze, right? Also, would the clear -leak stop liquid react with Dexcool to turn it black/dark red and make the temperature shoot up from 150 to 175 to almost 200? I had a flush done and the temp seems to be holding btw 150-160.