This is the effect you sometimes see on black and white posters of children with that slight touch of color. It's a pretty easy effect and it can be very beautiful for portraits of children or weddings.
Photoshop Tutorial. Selectivel Colorize an Image Part 1.
Learn one very simple way, using the mask of the Black & White Gradient Map Adjustment layer to conceal or reveal parts of the image in color and other parts as a black and white version .