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From my experience, the fastest way to do this in PhotoShop is to press
Ctrl+J (Duplicate layer) with the colour layer selected, and then with the layer copy selected, press Ctrl+U (Desaturate) after that, I just erase the parts I want to have coloured. Just switch the order of the layers and eras if you want it the other way around (i.e. you want to just have a part of the image in B/W).
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