Make one colour stand out in this selective colour photo effect
This can work with any image, but it does particularly on a subject that has a solid colour. To single out this colour, such as the yellow in these flowers, go to Select>Colour Range. The preview shows the selected area as white.
Select the colour from the Select Colour list or, if this hasn’t quite worked, set Select to Sampled Colours and click on the colour you want to keep. Use the Add to Sample tool to select every part of the subject.
Push fuzziness to around 80, or until there are no white areas outside the subject you want to colour. You can use the Subtract from Sample tool to further refine the edge until you’re happy with it, and then hit OK.
All that’s left to do now is to remove the colour. Go to Select>Inverse and then to Layer>New Adjustment Layer, choosing Hue/Saturation. In the adjustment, reduce Saturation down to -100. Only one colour should remain.