For this tutorial I will be using this picture:


Now go to your materials palette and go to the Gradient tab



Pick any gradient you'd like and press okay.
After that make a new raster layer by going to Layers > New raster layer. Flood fill that layer with the gradient that you picked and then set the entire layer to Lighten.

And you're all done! You can try this with different layer settings, gradients, or anything else you can think of.