MG TF 120 Overheating Problems

i have a 2005 MG TF, just replaced the head gasket, new radiator, new water temp sensor, new thermostat ... collected the car from the garage yesterday and this morning the coolant boiled over again. it's just ONE day from collecting the car from the garage! I opened up the coolant tank and realized that there is some brownish stuff stuck to the interior of the tank and cap. What might those be? Did we blow the head gasket again in this one day (the distance upon discovering the car was overheating and fully stopped is less than 2 mins so i would think this is a remote possibility)? Or are those remnants in the pipes that are now loosened up and deposited into the tank?

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