I cannot believe it, but there seems to be no easy way to turn the browser content to be b&w once for all with a shortcut to opt-out back to full-color on the particular page.
https://superuser.com/questions/330830/how-to-turn-web-pages-in-browser-into-black-white-grayscale
@johan last time I missed shit in the browser, I wrote the extension myself; gosh, it was 12 years ago and I hate doing it again.