Now this could be useful, or at the very least, fun. Selective colour is a popular technique where you take a colour image and turn all of it except for a particular object into black and white. There are various ways of doing it, which mostly involve making careful selections in Photoshop, fiddling with this, tweaking that, and generally messing around. People who do it a lot can no doubt get the look they’re after very quickly. But if you don’t want to spend a lot of time on that kind of thing, you might like ColorWash – currently on offer in the Mac App Store at just 59p. All you do is open an image, which the app converts to monochrome. You can then use a brush tool to paint the colour back in where you want it, or the fill tool, which seems to do a reasonable job of detecting edges and filling in a whole object. Here’s one I did as a quick test:
Better results could be gained with a bit of work, but if you just want to mess about a bit, this is ideal. And at the price, it’s well worth a try.
 Or your choice of image editing software