The Color Picker in the Theme Builder has three tabs (two when used to edit a Theme Color): Color, Web Color, and Theme Color.
The first tab shows a full-fledged color picker (a modified version of Color-Picker-with-Color-Wheel-and-Eye-Dropper) that lets you build a valid #RRGGBB or rgb(r,g,b) or rgba(r,g,b,a) color notation.
Drag the picker on any window to select a color from another application. User the Opacity and Lightness sliders to adjust the transparency level and the color brightness.
The second tab shows the list of all known, valid, and cross-browser Web Colors.
The third tab shows all the named colors defined in the theme itself.
