ColorChoke

ColorChoke 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. ColorChoke works with any image that can be opened on the Mac. You use 3 sliders to adjust the color key masking: the Threshold and Smoothing sliders in the inspector panel and the vertical color scale slider in the color control panel, and in addition you drag a marker around the color wheel to key on which color range you want to mask out, or drain, from your image. ColorChoke renders the effect instantly, and applies auto-cropping to the image, meaning what you see in the main window view is what gets saved when you save the image, you just resize the window to adjust the image size.