Why is my coolant rusty?

I've heard of guys filling their cooling systems with EvapoRust. Running the car up to temp and then letting it sit over night. In the morning, drain it out and flush the block with water to get the rest of the EvapoRust out. My cooling system looks the same as yours on my Challenger. New radiator core. It's the flash rust from inside the cooling passages of the block.(like Mike said)