DROPPER is a drawing tool that can be used to set the current primary or secondary color from a color of a clicked image pixel.
Simple click sets primary color and a click while the SHIFT key is down sets the secondary color.
While drawing tool like brush or rectangle are active, it is possible to change active color. When the CTRL key is down, the tool temporarily works as the dropper tool (the SHIFT key functions as usual).
Find out how Vista icons differ from XP icons.
See how RealWorld Icon Editor handles Vista icons.