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Photoshop
If you have Photoshop, it's dead easy. Create a new channel mixer adjustment layer, making sure to tick the 'greyscale' box to get a B&W effect. Then, with the new layer active, just erase out the bits you don't want to be in B&W, like you are scratching away at the top layer to reveal the colour underneath. If you make a slip, you can undo (ctrl-Z is quickest) or you can use the brush tool to go over the mistake.
Hope that helps.
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