My Android developer options has Colorspace, Red, Green, Blue, Grayscale. Last one puts everything as grayscale.
As a curiosity, I took some screenshots, and did some video calls, shot some photos. Everything on my phone was grayscale, but each of them on other devices, phones showed all normal colors.
Samsung phones have this built in. I really don't see the point of turning everything to grey?? But I'm of the age where I saved up for my own B/W TV with my newspaper job :)