I just cleaned the cylinder head mating surface with a die grinder wire brush did i screw up?

Pee poo , aluminum block material is much denser than many other aluminum types. the shade change is likely a minor discoloration from the cleaned material. One point though you needed to ask is what is the procedure for working with Aluminum engine blocks. when cleaning the surface of aluminum you never stay long in one spot! aluminum heats without much warning to distortion, steel will glow when getting hot but aluminum is nearly invisible till its too late.