I think this is a very clever solution to making tri-colour e-paper displays (EPDs):
(Youtube)
I am pretty sure the additional value of a fourth colour is very low. But I am wondering if a similar trick wouldn’t be possible with larger size negatively charged pigment particles of a fourth colour?
And what colour would be best? I see some tri-colour EPDs come as black/white/yellow instead of black/white/red but would green be suitable for an imagined quad-colour EPD?
Henning's blog 