No way to save a color in Preferences - may colors be properties?

[NSUserDefaults setObject:forKey:]: Attempt to insert non-property value ‘NSCalibratedRGBColorSpace 0.709417 1 0.940601 1’ of class ‘NSCalibratedRGBColor’

That’s right: you can only save stuff that can be stored in plists. You need to use NSKeyedArchiver and NSKeyedUnarchiver to save/load colors in user defaults.

Something like:

set theData to current application’s NSKeyedArchiver’s archivedDataWithRootObject_(myColorWell’s |color|()) ?

Yes – see “Storing NSColor in User Defaults” in the docs.