How do you design a rainbow in Photoshop?

Answer 1

Easy way:

Create an ellipse marquee, fill it with the last colour (ROY G BIV), violet. Next duplicate it and shrink it with free transform (Ctrl + T), hold Shift and scale it down. Fill this with the next colour, indigo. Do this until you have the full range in circles like a target. Then use the rectangle marquee tool to cut it in half high-wise and voila, you have a rainbow. Use masking with a soft brush to fade the ends and turn down the opacity of the layer.

Realistic way:

Google "Photoshop realistic rainbow". I found one probably suitable result instantly.