In this tutorial  I will explain  How to "Add Realistic Rainbow Effect" in Photoshop.  This is easy Photoshop tutorial, aimed at beginners with detailed explanation  at each step. I use Adobe Photoshop CS5, but you can follow all these steps  in   Photoshop CS3 to CC.

All step by step instruction  with images  are HERE. You can also download this Wallpaper, which shown as bellow.

You can select any wallpaper,  which you want to add rainbow effects.

Use Gradient Effect:  It provide six color such as Red,  Yellow, Green, Sky Blue,  Blue, Purple . 
Gaussian Blur Effect:  It prove the real effect on rainbow.

Wrap:  You can  Wrap as an "Arc".
Distort:  You can set the opacity of the selected area.

After follow all these steps, you can make best rainbow effect on any image.