Discolouration between breasts?

Hi, my wife is developing a heat rash like thing between her breasts now that they have gotten bigger for breast feeding. we live in the tropics and it is really hot and humid here right now. right in the middle of her chest between her cleavage is darker than usual and it is sometimes dry and itchy for her. Is there anything topical she can use? She is moisturizing there more now but i was just wondering if there was anything else she can use to help reduce it.