The ability to set different colors on character cells on different masters.
That is already possible. Select a glyph, go to context menu to find the glyph colour, and press alt/option.
Aha! A secret trick – the old option key. Thanks Tosche.
Also described in section 6.3.5 Color Label of the Glyphs 2.3 Handbook: https://glyphsapp.com/get-started
It turns out I already knew about the two-color cells but hadn’t considered using it to differentiate masters. It will work OK.