A custom color well button with a custom popover menu of colors to replace NSColorWell. Plays nicely with the OS X color picker for custom colors.
To use a JXColorButton, simply drop a custom view in and subclass it to the JXColorButton class and implement the JXColorButton delegate on your view controller or window controller. :)
You can customize the colors, dark/light background vibrancy of the popover, menu text, coloring, spacing, selection highlight colors, and a host of other options. The code is well documented, tested on OS X 10.11 (although it needs more testing), and written in Swift 2.