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?
Starting with the source image in greyscale:
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.
I say: Don't start with a greyscale image!
Use the layer effect "Gradient Overlay" with the blend mode "Color". That way you can keep the color information and add some depth by using only i.e. 50% opacity of the effect.
These are the steps I took using GIMP to create a color gradient on the source image.