To set up ‘red mode’ on an iPhone (iPhone X and above):

Go to Settings → Accessibility → Display and Text Size → Color Filters.

Enable Color Filters by tapping the toggle at the top of the screen.

Select ‘Color Tint’ as the filter type.

Slide the intensity and hue all the way to the right.

Now, go to Settings → Accessibility.

Scroll to the bottom and select ‘Accessibility Shortcut’

Choose the "Color Filters" option from the list.

Now you can triple-click the side button on your phone to switch Red Mode on and off.