Rainwater comes from the ocean, algae can be green black and red (and rarely others, most commonly red and green.)
I can tell you (I have photos of my fish tank with the water sometimes turning green from the algae) that rainwater CAN have algae in it to turn red/etc. Hell, water spouts have caused it to rain FISH deep inland and it is recognized that waterspouts cause this, just as absorbed algae in moisture in clouds can cause it to rain red or green rainwater. Do fish come from the moon!?