This used to work in Glyphs 2, but doesn’t seem to be working currently in Glyphs 3:
Whenever I duplicate a master, the layers (for instance color layers), aren’t duplicated along, and only the fallback glyphs is duplicated. This is quite troublesome when creating variable color fonts. There are multiple workarounds of course, but using the base UI kept it simple, also for my courses.
The drag+option method does work though.
Or do I have to change a setting/preference somewhere?