Colorize caters for the non-artists among us with almost idiot proof usability. To get started you simply drag-and-drop an image of your choosing and let Colorize desaturate it for you. You then re-saturate only the portions of the image you want to highlight using a paint brush-type tool.
You can vary the brush size using a slider, zoom in and out from the tool bar or using pinch-to-zoom, or switch between colorizing and 'uncolor' with nice large buttons on the toolbar. If you make a mistake you can hit the undo button, or save your image in its original size when you're done.
There's not much else to the app, which makes it incredibly simple and easy to use, but if you're looking for a boat load of advanced features, Colorize will disappoint. I also experienced an odd bug when testing the app, in that it wouldn't let me re-saturate the image at maximum zoom -- the brush tool simply didn't work -- but I'm sure that's a bug that can be easily fixed.
Colorize isn't the first selective color app in the Mac App Store, and it won't be the last. If you're a keen artist, then this isn't for you. But for those of us who can't afford Photoshop, or just want a quick and fun tool, this app will do the job.
Original photo credit: Jonathan Gill