Hi,
1)Duplicate the layer.
2)Desaturate the new layer
3)Create a mask on the new layer
4)Click on the mask
5)Select the brush. Change the color to black.
6)Feather the brush (This hides the hard edges as you brush).
7)Brush on the object or person you want to color. Since the brush is black this will hide the black and white pixels on the new layer, which reveals the colored pixels on the bottom layer.
-Rob