adobe photoshop - How to add a color gradient to a picture

I felt in love with the way this graphic designer, called Mariusz, has edited this picture.

It would be fun to produce something like this for myself.

Anyone has some ideas on how to create such a nice color effect in Photoshop or Gimp?

Answer

For Photoshop:

Starting with the source image in greyscale:

^{Source: http://everysinglepixel.com}

Add a layer with a gradient fill on top of it, adjust the blending mode for the layer to something that looks nice. I chose Soft Light and tweaked the opacity.