Rust tells you alot about the QUALITY of the cast iron poured to make your rotor.

The SAE acknowledges 9 different quality grades.
Here's a little rotor education but just an intro another few thousand pages of research and reading is necessary.
http://www.sae.org/events/bce/tutorial-ihm.pdf

I had my stock rotors resurfaced, and it turns into that color in same exact place. It is definitely normal.