iOS 11 stuck in greyscale
I guess I gave up too quickly when I mostly found web pages only talking about how to turn it on.  A few more minutes of searching found https://www.payetteforward.com/why-is-my-iphone-black-and-white-heres-the-real-fix/ and reading all the way to the bottom.

I don't know how it happened, but 'zoom filter' was set to greyscale &amp; invert colors, and the filter was being applied even when things weren't zoomed in.  (and I wouldn't have set it, as I use zoom for lots of things)

But it seems to use a different mechanism than the other accessibility options, so you can't disable it from there, you have to go into the zoom settings.