1. In your image-editing software, create a duplicate layer of your photo (Control [or Command] + J). Convert one layer to black-and-whtie by going to Image>>Adjustments>>Channel Mixer>>Monochrome.







2. With the black-and-white layer highlighted, create a layer mask by choosing Layer>>Layer Mask>>Reveal All.










3. Choose the eraser tool and set the foreground color to white. Click and drag to "erase" the black-and-white layer, letting the color peek through. Merge the layers by selecting Layers>>Merge Down.