macOS How to pick a color?

In the DragItemAround sample code from ADC, there is also a color picker that can change the color of the rectangle. I see the method that does this, but where is it connected in IB or wherever it is connected. I can not see how it gets invoked, and I want to have similar functionality on my application.

I have looked through documentation, sample code, etc., but I am still convinced that IB is magic!


EDIT:::::
Nevermind, the showColors method is a delegate method.
Sorry if you wasted your time reading this!