I agree with the others. Your roof is already white.
However, if you really want to do a roof coating, you need to be carefull of which product you use. Some of those coating materials, can actually damage your roof. Others may just peel off. Petroleum distilates don't play nice with rubber roofing.
Do a forum search for "roof coating". There are other threads about doing this. Some point out good products, and some point out some not so good.