My city row house is 100+ years old and has a low pitch modified bitumen roof. It looks like a one piece asphalt or tar sheet covering of the roof. I bought the house 7 years ago and never had it treated, I think it looks like it hasn't been done in at least 10 years. The roof has no leaks but some little cracks in the cover and the paler color than the neighbor's indicate that the time is due perhaps.
The current covering is silver with some patches of black surfacing up. I imagine the black is indicative of wear and tear and the silver is the original, healthy finish. At least the neighbor's is all solid silver.
I noticed that the brand Henry at the Home Depot has several grades of tar/asphalt roof covering that comes in 5 gal buckets. I was wondering which grade to use to apply over my roof. I don't think any of them match the current silver of my and my neighbor's roofs though. The roof area is approx 900 sq ft.