What can I do if my phone screen is yellowing?

Check whether “Eye comfort mode” is active. Eye comfort mode will filter out blue light
and adjust the colour temperature of your display to reduce eye strain.
1) Go to “Settings”->”Display”-> “Eye comfort mode”-> deactivate “turn on now”.
2) Drag down quick settings bar in any interface ->touch “Edit” to add "Eye comfort mode” icon to activate/deactivate it (slide to the left to view it in the second page).