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 pallette
Choose Rainbow gradient from the link and draw a line on your image
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjLGCinOviBnuV0xEJjAZX0skl9iZxOmUPCtR1FccQyJSXX-_JU-2BQjIwZnNRrXWtfu6puSe0vMtyh1Ze-guhgBVb3ApGi7fzs3Fua065wvk0ZbeyciZSRfn-iX1ioFnRBSnC1b0IcZGI/s1600/3.png)
Then the whole image will be covered with Rainbow effect
Now change blending mode to Overlay
Well as simple as that
cheers!