All this doesnt explain the color though.

that color seems like it may be stuff from the radiator like was said.

How much did you clean/flush the radiator before use? Thats kind of the color the first few empties of hot water looked like when I flushed (it wasnt as dark though, but was in there for only a few seconds)

That or some sort of rust in your loop, dunno how that could happen, would need to be a decent about to cause color change, so thats prob not it.

Some sort of crazy red algie, but would most likely see buildup on the tubes.

or as was also said, some kind of red dye that was used, and not fully cleaned out of one of the parts, now bleeding back in. Any of those parts used? or have you used them before with dye?