Thanks to Damian for these instructions

Andy

1. Settings
2. General – accessibility shortcut on right at bottom
3. Colour filters – tick on
4. General – display accommodations
5. Colour filters tick on
6. Choose colour tint
7. Touch finger on red crayon
8. Increase intensity right up

Once set up the red colour can be turned on and off by clicking home button 3x in quick succession.