I agree the lemon juice will help bleach out the brown color, but what is causing this problem? Do you have calloused elbows from lying on the floor (playing video games, doing paperwork, etc.)? If you are calloused, you may also need to use a fine pumice stone (rub gently) after your bath or shower and then rub heavy-duty lotion into your elbows. My son had this when he was young and the doctor laughingly called it Ninento Elbow. Anyway, it may take a little work, but you'll get rid of it, I'm sure.
Soibhan