There are a number of decent convertible top cleaners - anything from an auto parts store should work OK - they all seem to work equally well as long as you don't let a bird stain or oil stain sit for too long - but don't get aggressive with scrub brushing. Black is pretty easy to keep clean. Soft bristle brush, rinse very well.
You ought to see a white top after getting caught on the interstate behind hundreds of diesel trucks for a couple hundred miles in a pounding rain storm....