Why is "pelo castaña" incorrect?
A. This sentence is actually correct.
B. The adjective (castaña) is not in agreement with the
noun (pelo)
C. It is talking about a woman's hair, therefore, "pelo"
should be "pela."

I'm actually not sure about this one because pelo castaño sounds better to me than pelo castaña, I searched it up as well and corrected it to castaño. It is either correct or it is B. Like I said I don't really know even though i'm a native speaker but I hope this helps a bit !