Create powerful imagery by singling out colour using a nifty selection
Go to the Select menu and down to Colour Range. Tick the Localised Colour Clusters box and use the Eyedroppers on your image to pick out the colour to keep. Boost Fuzziness to 150% and Range 100%. Press OK when the colour shows up white in the image’s preview.
Press Ctrl/Cmd+Shift+I to inverse the selection and then add the Black & White adjustment layer from inside the Layers palette. If any areas show through in colour that you don’t want appearing, use the Eraser tool on the adjustment’s mask.
The advantage of using this adjustment layer is that it comes with preset monochrome styles. For portrait images, use the Maximum White preset. For landscapes and cityscapes, try out the Green filter.