This is an accessibility feature for people who have difficulty seeing certain objects on their screen due to colorblindness, but some people claim that turning off the bright colors on your phone has the added effect of making you less addicted to using it. The theory is that basically every app is designed with just the right combination of colors and notifications to get you to use them (it’s basically impossible not to tap on something that has a big red notification number on it, right?), so taking away the colors will wean you off of them.
You can apply this filter under Settings > General > Accessibility > Display Accommodations > Color Filters. There are also options for other various types of colorblindness.
If you want to be able to switch back and forth quickly, you can go to Settings > General > Accessibility > Accessibility Shortcut > Color Filters and activate the shortcut. After that, you’ll be able to click the home button three times to turn the filter on or off.