ColorDrain

ColorDrain is a color masking app that allows you to highlight certain color ranges in your images and leave the rest of the image as black and white.

It works with any image that can be opened on the Mac. You use 3 sliders to adjust the color key masking. This tool renders the effect instantly, and applies auto-cropping to the image, meaning that what you see in the main window view is what gets saved when you save the image.