Make it black and white
Image- Adjustments - Black $ white
Make a new Layer and name it anything you like
With a new Layer selected, choose gradient tool from tool palette
Choose Rainbow gradient from the list and draw a line on your image
Select reflected Gradient
Then the whole image will be covered with Rainbow effect
Now change blending mode to Overlay
Take Erase tool and erase on areas where you don't want rainbow to appear
well that's it
