[there4]

Wired suggested using grayscale mode to help make the phone less engaging. On iOS, I believe you can find it under Accessibility > Display & Text Size > Color Filters.

https://www.wired.com/story/grayscale-ios-android-smartphone...

[executive]

Seconded. On iPhone 8, you can also set it to enable/disable Color Filters via triple tapping the home button.

For eye strain, use the Reduce White Point setting combined with minimum brightness at night.