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In the CNET video below it looks like a different home screen and everything you launch into, it doesn’t just switch to monochrome and both articles claim massive battery life gains when in eink mode meaning it’s not just LED but curious to know more and keep an eye out
Afaik it’s an LCD that can use more reflected (environment) light and not just it’s own. So yes, high energy savings compared to having the LED background on at full.
Over the decades a few such LCDs were developed by various companies, nifty tech & experience, but the growth of battery capacities allowed for punchier colors of classic background LED LCDs and now OLEDs to dominate.