I've heard both ways - some that swear it will harm and some that swear it won't. Depending on your hair of course you can give yourself very natural highlights using lemon juice diluted with water and then let your hair dry in the sun. I'm a natural light blonde anyway so I just use straight lemon juice when I want to lighten it a bit. Oh, and chamomile tea (pour it over your hair, using a bowl to catch the drips and repeat several times with what drips into the bowl) is supposed to brighten hair for brunettes - I tried this once, and although it made my hair very, very soft, it did darken it a bit
Good luck!