Bonnie IS a natural red-head. The Weasleys are supposed to have startling red hair, nothing subtle about it! Rupert has it naturally, the twins who have dark brown hair have an out and out dye job. It's ironic that Bonnie has to have a wash to make her hair redder as opposed to her natural, more auburn shade. Still, as she's a born redhead, it does mean that her skin tone and eyes suit either subtle or bright red hair.

emerald is right, she does get the colour from her Dad. It's hard to compare her gorgeous locks to his, er, light reflecting shine, but if you look at his right (to our left) eyebrow, you can definitely see it!