1994 Ford Mustang

I had a problem with my car overheating. I had no heat in the vehicle as well. The coolant had boiled over through the radiator cap. When I had checked the coolant in the radiator and the over-flow it was a brown rust color. There is also a build up of gunk in the system with the same consistancy of whip cream. It is also brown in color. My oil level is fine as well as transmission fluid. I had taken it to the shop and was told to replace the radiator and water pump because they were clogged as well as flush the system. I have done all of this and am now just waiting for time to finish. The old radiator was only a year old. What caused my coolant to do this? I have heard that it is from two different colored coolants mixing.