The feature you are asking is not supported.
You can achieve it by doing some tweak. Here is how you can do.
Create a transparent gradient button similar sized to the colorwell and arrange them as shown in this pic.
Then write an action for the button itself. User will feel that he is clicking on the colorwell.
- (IBAction)showColor:(id)sender {
[[NSApplication sharedApplication] orderFrontColorPanel:nil];
}