iPhone XR
My screen became black and white suddenly. Cant find a way to get back to color.
Question marked as
Try triple clicking the side button, and make sure Color Filters is not turned on.
Alternatively go to Settings->General->Accessibility->Display Accommodations->Color Filters.
You can also go to Settings->General->Accessibility Shortcut, and set what you want the triple click to do. More details here: About the Accessibility Shortcut for iPhone, iPad, and iPod touch - Apple Support