Colour Spectrum background

To create this image I started with a Gradient from black to white, I then went to filter distort and wave and set the number of generators to 10 and the type to square. I then added another gradient overlay to the effect and used the Rainbow Gradient with an angle of 0 degrees and the blend mode to linear burn and put the opacity to 80%. On a new layer I then added another new layer with a gradient from black to white and set the blend mode to soft light. I then added the little squares as a brush and painted over the image on a new layer and use the blend mode overlay to add the finishing touch.