This photo manipulation app has a simple function. It allows you to convert your photos into Black & White versions, but you isolate a specific color you want to keep. It can have a very artistic effect on the right type of photo. It can also be a good way to draw the viewers eye to a specific place in the photo, similar to how photographers play with the field of focus in a shot. This app works best on photos with strong composition, and has one particularly bold color in the original. The app offers slider controls for setting the sensitivity range of the color selected, as well as brightness and contrast.
Here is a sample image alteration performed with this app: