[mtklein]

If you're an Apple user, it's built-in, and I've found it ergonomic to assign its toggle to a triple-tap on your phone's back and/or laptop power button.

Those triple-taps are effortlessly quick and rarely if ever misfire, making it totally reasonable to stay in greyscale by default and reengage color selectively when you're looking at something that merits it (film, family, friends, a Wordle scorecard...)

[RMPR]

On Android it's also built-in, iirc it's called Bedtime mode.