Stain is an app that penetrates the inside of contours within your photos or drawings. Tap to easily coor areas that are difficult to paint using your fingertips. The color spreads more and more when you leave your finger on the screen and adjusts the spread according to the length of time you first hold down your finger. It’s enjoyable to use, from minor tone correction to very detailed work. This app retails for $1.99 and you can pick it up here.