Simply call the following line at any time in your app.
[[NSColorPanel sharedColorPanel] setShowsAlpha:YES];
You can call it once in applicationDidFinishLaunching:
or you can easily tie it to a switch like an NSButton checkbox with a simple IBAction method like this:
- (IBAction)showAlphaSliderInColorPanel:(id)sender {
if ([sender state] == NSOnState) {
[[NSColorPanel sharedColorPanel] setShowsAlpha:YES];
} else {
[[NSColorPanel sharedColorPanel] setShowsAlpha:NO];
}
}
Just connect that to the Sent Actions selector item in the Connections Inspector with for a button configured to have an on / off state.
The change will occur live as you click.
A great example of how awesome Cocoa is when you want it to be.