If you have a solid barrel(no light gets through) it should not turn green. If you free range chickens will drink out of mudd puddles, so I would still allow them to drink water off roof. Animals drink water from algea infested, rain runoff filled ponds all the time, so unless your putting something into the barrels other than the rain water it is fine for your chickens. Have you ever seen the mudddy dirty water that a duck drinks?? If rain water was bad for fowl we would have no surviveing ducks.
