Some more findings below: It seems that Apple supports the DeviceN color space model but not the Separation color space mode. The DeviceN model appears to support the concept of Spot Colors but I can't seem to find any details on how one much create a DeviceN color space using the Apple APIs. https://developer.apple.com/documentation/coregraphics/cgcolorspacemodel

Can you elaborate a bit more on how you did this. I tried adding the code above to my AppDelegate and it seems to make no difference - the main menu remains as it was regardless of the contents of the MainMenu nib. For example what is this line for and do you bind that to a window, and if so where ? @IBOutlet var window: NSWindow!