Desaturating a color image is easy enough. Just press CTR+SHIFT+U. Restoring color back into a desaturated image is also possible and quite easy. What I intend to show in this Photoshop tutorial is how to blend the desaturated part with parts still remaining in color, seamlessly.

I have taken this free stock photo.

Desaturated it.
Restored color in parts and blended it with the desaturated portions seamlessly.
The tutorial is here.
Other Photoshop tutorials are here.