ColorPickerPlus replaces the standard Color Picker to provide

color specification through a hue bar and gradient box
text entry for colors (RGB, hex, and HSV)
text entry for the alpha value (transparency/opacity)
a 36-swatch color palette that can be used for copy/paste
a class color palette
a copy/paste independent of palettes
a movable frame
a color swatch for the color to be replaced
a color swatch for the chosen color

With these enhancements, you can precisely describe a color and coordinate colors within and between add-ons.

Left-click on a palette swatch will choose that color.

Shift-left-click on a palette swatch will save the chosen color into the palette.

To switch between the customizable palette and the class color palette, click the button labelled "@".

Control-left-click on the chosen color will print the decimal RGB values for the color into the chat window. This was added as a utility for add-on developers.

Useful external tools for generating color palettes:

https://color.adobe.com/ and http://tools.medialab.sciences-po.fr/iwanthue/