The application lets you select different areas of the image and give them each different effects. For example, you can change the color of the sky in one image and nothing else, or apply an effect to only one person in the picture.
You can play around with the image settings and get pretty great results. You can also check the 'before' and 'after' comparison at any time to see what each effect does to the photo.
Magic Colors is a pretty useful tool. Although it doesn't offer too many features, it makes up for that by being extremely easy to use.