Add opacity slider to the color panel for one color well but not others

I'd like to add a opacity slider to the NSColorPanel that is shown for 1 specific NSColorWell. All other color wells should not show the opacity slider.

I know I can set this for the sharedColorPanel like so:

 [[NSColorPanel sharedColorPanel] setShowsAlpha:YES];

But how do I do this when I only want this behavior for a single color well?

I tried adding an IBAction, but this IBAction is not called when you click the color well. (So I can't make any changes before the panel is displayed). It is called when you choose another color in the color panel.