One note about the color of the rain barrel. A dark color discourages algae growth, but if the barrel sits in the sun, the water will heat up during the summer. I have two 60-gallon / 227-liter barrels in dark green, and sunlight hits them from dawn until 1 PM. Two weeks ago, after I finished mowing the lawn, I put my hand on top of one and found that it was uncomfortably hot. My infrared thermometer said the lid was 150°F / 65°C.
That heat gets transferred inside: in July and August, the water at the top will heat up to 110°F / 43°C. That isn't a water temperature that I want to put on my plants. I have to mix the water from bottom to top to cool it down to something less ridiculous. I'm planning to make a couple sunshades that I can put around and over the barrels to keep the water cooler during warm weather.