An icon generated from image is not always perfect. In some cases, it might be helpful to make additional modifications to repair an imperfection or emphasize a feature.
The Retouch tool allows application of different effects to a region specified by another tool (by default Brush). Effects can be applied with various strength, even with values outside the 0-100% range. The retouch tool can be used to:
Pencil tool can be used change colors of individual pixels. Hold down CTRL key to temporary switch to the Dropper tool. The Replace drawing mode is useful when changing colors of semitransparent pixels.
Use subtlety when changing icons, every pixel of a 16x16 image counts and a small change have large effect on the final impression.