If you use MacOS, you can get some of that black-and-white feeling back by going into System Settings > Accessibility > Display, scroll down to Colour Filters, turn it on, change Filter type to Greyscale and set Intensity to High. I do this on my personal computer to try and make it less compulsive at weekends, to remind me that it's just a tool, and that there are other interesting things to do with my time. I'm probably getting old, but I also find this more restful to look at. Sometimes you need the colours because apps rely on it: although I can't remember how I did it now you can add an Accessibility Shortcuts item to the menubar which allows you to turn the Colour Filters on and off with a click, drag and release on Colour Filters.

I also use that filter on iOS to tone down the colours by setting the Intensity of the same filter at a much lower level - I find the standard colours really garish when I turn them back on (e.g. when looking at photos). On iOS you can set a three-click shortcut on the action button to turn that on and off.

Edit: I forgot, there's also an "Increase contrast" setting there which makes the UI even closer to the older MacOS look, but although it does give different areas more differentiation I find it a bit too harsh - I think because it's just flat black and white, whereas the older systems used more greyscale textures.