Get yourself a fabric marker, black, and put a coupla coats on it. Just keep marking away until you get it nicely blackened. Every year or so touch it up. Your's looks to be pretty solid still, I would just follow my (proven here) advice!
And I don't believe the cloth is even necessary ; I have seen it removed, and the same marker used to darken the rubber underneath. Either way, no problem with the windows that I've seen. You
CAN peel that cloth off and be done with it.....