an easy way to do what you're asking for is to add a hue/saturation adjustment layer above your color layer.
When the dialogue box comes up - slide the saturation all the way down until the image is completely black and white.
Then - on the adj. layer in the layer's palette - you'll see a white box to the direct right of the adj. symbol box. Click it so it's selected. Then hit 'b' for brush and paint with black paint on an area you want to be color again.
That is basically layer-masking out the 'gray' effect and letting the source layer show through.