There's also an increasing prevalence and popularity of e-ink displays. Though some of these are colour-capable, most are monochrome, and I've definitely had challenges negotiating apps and content which use colour as a key differentiator.
Personally, I think it's just that monochrome icons are fashionable. I can't imagine that the GIMP developers for example chose to make GIMP's icons monochrome because that works better on e-ink screens.
For text and graphics, you'd generally want to use a higher-quality, lower-refresh-rate mode. Which is to say that there's quite a bit of established UI/UX knowledge that wants a refresh. I've distilled a set of basic principles: persistence is free, pixels are cheap, paints are slow, colour is (mostly) nonexistent, and the more ambient light the better. Paginated-navigation (whole screen changes in one go), line-art, and dithered or halftoned images work relatively well, raster images not quite so much, though can be acceptable.
<https://diaspora.glasswings.com/posts/638a8d10e041013afba844...>
For a demonstration of a wide range of displays (the best ones are featured last) as of 2021, see: <https://yewtu.be/watch?v=KdrMjnYAap4>
... I'm having difficulty finding e-ink / electronic paper market research, though one hit suggests an 8.5% CAGR: <https://dataintelo.com/report/global-e-ink-sales-market/>.
Using HN as a rough proxy, I see fairly substantial growth. Note that HN submissions overall have been fairly constant since 2012, per dang.
"e-ink"
2007: 1
2008: 7
2009: 14
2010: 18
2011: 24
2012: 28
2013: 24
2014: 24
2015: 16
2016: 25
2017: 22
2018: 39
2019: 35
2020: 60
2021: 88
2022: 84
"Digital paper" has an even smaller smattering of hits.
... though Apple has had monochrome devices (the original Mac, the handheld Newton) ...
... and often makes preparations years on advance for long-term strategic moves.
I'm putting very low emphasis on that last, as e-ink really doesn't seem to fit particularly well with the Apple ethos, except, perhaps, for watches.
But if Apple were planning, or even contemplating, an e-ink device, ordering a flat white UI/UX might be precisely the first step in getting there.
Many of these issues already exist, e.g., for documents which are likely to be printed on monochrome devices. Colour is useful where available, but it is not universally available.
(And that's before addressing issues such as colourblindness or other disability / accessibility considerations.)
My original comment was not at all specific to Apple (you'd introduced them to the discussion), other vendors, or any specific OS or application. Simply that monochrome and e-ink are increasingly concerns.
But since you raised the topic of Apple, and since, contrary to your initial statement that, broadly speaking, monochrome was "a display technology they never used", I'm both correcting the record, and noting that should there be a monochrome or e-ink prospect on Apple's roadmap, and further noting that Apple rather famously does not announce such roadmaps in advance, that flat/white would in fact fit with e-ink remarkably well.
What that has to say about whether or not Apple is contemplating any such move I am, of course, entirely ignorant.
Any explanation of the anti color trend must explain Apple's participation. Especially because Microsoft and GNOME appeared to imitate Apple.
I did not say monochrome was a display technology Apple never used. You named e ink. And monochrome is not a display technology.
Other trends included more transparency in content areas, links almost unidentifiable without color, and other reliance on small shade differences. And Apple made all app icons the same shape. All worse for e ink.
OK, that does it. You're a Hong Kong viking, not a Seattle baker.