If you paint them black they will heat the water, (if they are standing in the sun) this might not be good for the plants. I’ve killed grass and plants with the hose & solar heat. You could build a shelter with a few pieces of plywood to shade them and achieve the same thing.
I have’t got around to this yet but I want to place a tank in my attic and connect it my domestic water system. Then the gravity will provide pressure to flush my toilets and use the sinks. I have the cold water to the kitchen sink isolated from the rest of the system now, so if I set up a filter system I could use the 55 gallon food grade drum I bought for drinking water and install it in the attic as well. The water tanks can be heated with solar, furnace or stove for washing or to prevent freezing in the winter.
A lot of dirt comes off a roof system so I think pre-filtering before it enters the tanks would be desirable. I use poly bags in 55 gal drums in my studio for filtration and think they would be useful for anyone that wants to remove debris from a water collection system. The bags resemble a feed bag: Inner polyethylene liner bag
6.5 oz. woven polypro
2200 lbs. weight capacity
For 60 gallon cyclone
24″ x 42″
In my system a use two drums in series similar to the picture Whirlybird posted. I put a filter in the first drum and let water transfer from one to the other near the top of the drums. When the bag is full I replace it. In a domestic system without a crane, I would replace it based on a schedule so it could be handled manually.