The Adobe Photoshop Express app has a feature under Looks called Splash. This allows you to desaturate an entire image except for a certain colour range. It has options to leave in colours such orange, blue, green, aqua, red, RGB, yellow, purple and magenta. To achieve the same results in the desktop versions of Photoshop appears to be a much more convoluted process. This is an example of yet another very useful app version feature being omitted from the desktop application itself!