I tried that for a week. Couldn't get used to it. I found that too many apps use color for important distinctions, which they shouldn't because of colorblind people, but it is what it is.
My Nexus 5x doesn't have option to enable gray-scale, but I tried color correction settings for color blindness. Colors are there, but a bit washed out for Protanomaly. Will see how it goes.
It would be great if apps could know the display is in grayscale mode (perhaps it is?) so they could switch to a more appropriate design.