Go Gray with Google Digital Wellness
The Digital Wellness Application by Google was released for all phones with Android 9.0 Pie in November of 2018. The application features built-in grayscale mode and a grayscale toggle shortcut. For phones (Android 5.0 - 8.0), developer mode grayscale method is still a viable option for going gray.
Setting up grayscale toggle in Digital Wellness:
Step 1): Download the Digital Wellness here.
Step 2): Swipe down the top menu & edit click the edit icon.
Step 3) Drag Grayscale icon to desired spot.
Go Gray with Developer Mode
For Android phones with OS 5.0 - 8.0.
Step 1) Turn on developer mode. To turn on developer mode, go to Open Settings > System > About phone.
Then find "build number" (usually one of the last items in the list) and repeatedly click it 7 times. You should see a bubble that pops up indicating developer mode is now active.
Step 2) Back in the system menu, there is now developer options button. Inside developer options, scroll down to the hardware accelerated rendering section, find "simulate color space" to turn on grayscale