The rainbow contains as we know six colors. We will star by creatinga new document 400×400 pixels. There are many ways of drawing a rainbow, with the Pen Tool for example. But i will use another tehnique, the easiest i guess: 1. with the Elipse Tool draw a circle with color red. You need to right click the layer and choose Blending Options; Stroke 1px and the color light orange. 2. duplicate this layer - go to Edit - Transform Path - Scale and do the following settings :for the width 85% and height 85%; now color this circle orange 3.duplicate this layer, scale againg with 85% and choose yellow color 4.do the same with the other layers. the colors for the rainbow are: red, orange, yellow, green, blue, purple. You will obtain something like this:
Now select all six layers and scale them with width 120% and height 140%. Now duplicate the purple layer and scale it 80% both with and height and make it white. Select all the layers, right click and choose Merge layers.
With the Polygonal Lasso tool select and than delete the bottom part of the rainbow.
To give it some contrast choose Burn Tool and burn the lines of every color....take the Dodge tool and give some light areas.
Right click on the layer and Choose Blending options and do the following settings.
Now play with opacity, i choose 85 % and this is the result