1: Pick your shot
First, you’ll need to choose a photo to use. Pick carefully – it works best with shots that have a single colour featuring heavily in the foreground of the shot.
2: Add Adjustment
Choose Add Adjustment and select Black and White from the list. Choosing this rather than Desaturate will give you much more control over colours later on.
3: Brush away
Click the cog icon next to the brush options and choose Brush Black & White away. This will let you remove the effect in selected areas using an intuitive eraser.
4: Rough outline
Change the eraser Softness to the minimum and alter the size of the eraser itself to a sensible size. Then, roughly mark out the area that you want to be in colour.
5: Tidy it up
Zoom in and increase the Softness slightly, then start carefully drawing in any areas you’ve missed. Do your best to stay within the lines, but don’t worry about mistakes.
To correct areas that you have colour isolated accidentally, such as over the edges, use the Brush option to simply re-apply the black and white effect.
7: Adjust Saturation
Now you can start stylising your image. Use the Saturation slider to desaturate the colour in your shot – as there is only one left, the main colour will fade as a result.
8: Vibrancy and Contrast
Now use the Contrast controls to create deeper blacks, and the Vibrancy slider to push the brightness of the colours back up, until you find a nice balance.
9: Clean up
When the Vibrancy is increased, you may spot other colours creeping through – use the Black & White brush again to apply the effect to those affected areas of your image.
Click Image to Enlarge: