I will be using this picture:


Go to Layers > New raster layer
Now fill that raster layer with a gradient of your choice by clicking on your material palette

Then going to go the Gradient tab.

Now select any gradient you want to use, if you dont know how to create gradients, click here to learn how.

After you're done, fill your raster layer with that gradiant and set the layer to Hue (L) or Hue. You can decide which looks better. And it's that easy!