Two possibilities:
1.) If you want the original colors, open the original, copy/paste as a new layer into your B+W image (below the B+W layer), then on the B+W layer create a layer mask (reveal all), then paint on the mask with black to reveal whatever you want of the underlying (color) layer. Now if you want to change colors, you can just mess with the color on the bottom layer, using any of a number of techniques, e.g. channel mixer, hue/sat, etc.
2.) Convert the image back to RGB (still B+W, but now technically a color image). Select the area to be colored, and use the channel mixer to change the color.
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