Why does bleach turn my tub brown?

I use the Clorox foaming bathroom bleach. I sprayed it on a few areas that appeared to be less than clean on the tile wall. It turned the grout white. However, the bathtub itself is turning a reddish brown. It is not rust, the bathtub is the modern type, most likely fiberglass, but definitely not cast iron. I just spray it and leave it on without scrubbing or wiping it off. Thanks.