lemmy congrats on trying to use more mother nature. the roof would be fine no matter what material it is (shingles, metal, vinyl, terra cotta) and it's great stuff for plants. the only thing people generally worry about is called 'first flush'. the first few minutes of a rain storm flushes lots of unwanted stuff from the roof such as dead bugs, bird shit, sticks, loose roof debris, etc. the reason they care is usually they go on to further process it. you don't have to worry about it so much maybe just wait a few minutes then start collecting. less twigs and crap to deal with. the discoloration is tanic water with just a bunch of minerals dissolved.99% of time is fine but you never know.bacteria can be an issue...should be great water think about it, wouldn't all of nature show signs of problems? that's the only source they have.i really want to get more involved in 'rain harvesting' texas summers are really brutal with not much rain but you can get 1000 gallons per month from an avg house roof.