A color well lets people adjust the color of text, shapes, guides, and other onscreen elements. In Keynote, for example, the Rulers preference pane includes two color wells for specifying alignment and master guide colors.
When a color well is clicked, it darkens to provide visual confirmation that it's active. It then opens the system's color panel so the user can choose a color. Once a selection is made, the color well updates to show the new color.
Color wells also support drag and drop. Colors can be dragged from one color well to another, and from the color panel to a color well.