If the screen of your iPhone has suddenly turned black and white, the most common reason is due to the Grey Scale Filter being accidentally switched on your device.
It could also be due to someone in your home trying to prank you by switching on the Grey Scale display, which prevents iPhone from showing its regular bright and colored display.
iPhone Turned Black and White
Unless the problem is not hardware related, you should be able to fix the Black & White display on your iPhone by simply disabling the Grey Scale Filter on your iPhone.
1. Switch OFF Grayscale Filter
If the display on your iPhone is stuck in Black & White Mode and not displaying other colors, make sure that Color Filters are switched OFF on your device.
1. Go to Settings > Accessibility > Display & Text Size > Color Filters.
2. On the next screen, move the Slider next to Color Filters to OFF position.
Once you toggle OFF Color Filters, you should immediately find your iPhone switching back to its usual bright screen and colorful display.
2. Turn OFF Zoom and Gray Scale Filters
If the above method did not work, it is likely that you may have accidentally set Accessibility Zoom Filter on your device to Grayscale setting.
1. Go to Settings > Accessibility > Zoom Filter.
2. On the next screen, select the None option.
Note: This setting enables Greyscale Filter, only when Zoom Mode is enabled on your device. It has no impact, as long as the Zoom Mode is switched OFF on your device.