I believe that the calf will get darker with age and eventually be pretty well black in several months. Nice cow and calf by the way. I would also guess that she isn't a purebred Hereford. I have a few cows that are marked similar to her, but with a little different look as far as ear and head shape, that are part Santa Gertrudis. They sometimes have calves that are born that color and solid or with almost no white on the face.
I agree with Bigfoot, she looks like what we call a red neck here too, and they are usually pretty good cattle.