The best way to do it is to make a duplicate layer,

desaturate that layer (Hue/Saturation)

add a layer mask to it (if you look in the layers palette you'll find a button at the bottom, second one in)

Click on the layer mask (shown to the right of the new layer in the layers palette). Now you're free to add or remove colour using whichever tool you prefer (paint brush, magic wand, path selections, etc.)

Tip: when using the paint brush, reset the swatches at the bottom of the tools palette to black and white, then switch between the two using the 'x' key to add or remove colour.

When finished flatten the layers and save.

Alternatively you could just leave the poor picture alone!