Are you sure it is rubber and not vinyl? In either case I have not had good luck painting either with enamel or any other paint. What I do now with flexible tracks is weather with pastels.
For those using pastels, if you are not aware of jeweler's rouge look into it. It is real rust- iron oxide, ground superfine. It is used for polishing metals. It is much cheaper than hobby pastels on a gram by gram basis, though you are likely to have to buy it in quantity, like a pint or so. It applies like any other pastel. I do have a hard time finding a very dark gray pastel, which is a color I like for base on treads.