To make rainbow in Little Alchemy, you will need to combine the following elements: air, fire, water, and earth.
Begin by dragging two air elements onto the playing board. This will create a cloud. Next, drag the fire element onto the playing board. This will create lightning.
Then, drag the water element onto the playing board. This will create rain. Finally, drag the earth element onto the playing board. This will create a rainbow.
The four elements, Air, Fire, Water, and Earth, must be combined in the precise order to form a rainbow. Each of these elements can also be combined with other elements to form a variety of interesting items.