Pro tip: add it to the triple home button click (Settings>General>Accessibility>Accessibility Shortcut (all the way at the bottom)>Select 'Color Filters') for easy switching for color/grayscale.
"Here's a solution! Go to "about device", find "build number", and tap it a bunch of times until the phone tells you you are a developer. Then, go back to settings, and above about device, you will see a new option called "developer options". Open it, and find "Simulate color space", and set it to "Monochrome"."
Ref: https://forums.androidcentral.com/ask-question/686329-why-th...
BTW, I had to vouch for your comment to get it to not be [dead].