The rubber roof on my TC was pretty black from sitting on the Wet side of WA for years before we bought it this year.
Just went up last week and cleaned it. Bucket of dish soap water and a floor broom, scrubbed it all. Hit the whole thing with the 2500psi pressure washer. Came out pretty clean, still some stains but no more black streaks for a while. Wasnt slippery either like laundry detergent. That stuff is death on a roof! (Was spreadign Tide on my house roof in WA to kill the moss and it started raining. Almost came off the roof the quick way before I got done!