Leave just one colour in a picture using simple selecting tools
Start by grabbing whichever Lasso tool you work best with, and make a rough outline around the outside of the object or objects you wish to isolate the colour of. We’re going to leave nothing but yellow in this particular picture. Ctrl/right-click and hit Layer Via Copy.
Grab the Magic Wand (W), set the Tolerance to 25 and uncheck the Contiguous option. Click on New along the bottom, and click over all the shades of yellow until you’ve selected all of the yellow in the picture. When done, hit the Mask icon.
Duplicate the background layer and desaturate the colour by hitting Cmd/Ctrl+Shift+U. On the mask layer, select a brush and using black and white, touch up the edges of the colour in your picture to refine the selection even further.
Merge everything into one layer by hitting Cmd/Ctrl+Opt/Alt+Shift+E. Bring up the Levels palette by hitting Cmd/Ctrl+L, and use the stoppers to increase the contrast of the picture for a more cinematic shot.