Rain clouds are small greyish clouds with little arms and a face that constantly rain a steady stream of water downwards.
Rain clouds will follow the right eye, at a slightly slower pace then the right eye can walk, inside a particular area. They will constantly spew water downwards, and if the right eye touches the water, it loses all the colours it has gathered.
They often form dangerous hazards that must be evaded, or core elements in certain puzzles. They cannot be killed in any way. Notably, the first encountered rain cloud in the game - encountered on level three - does not follow the player.