Note!!! Only for OnePlus 3T devices with a S6E3FA5 display panel. See below XDA link below for instructions on how to check.
This runs the following command in the "late_start" service:
echo 1 > sys/devices/virtual/graphics/fb0/DCI_P3
This was taken from https://forum.xda-developers.com/oneplus-3t/how-to/enable-dci-p3-op3t-s6e3fa5-display-panel-t3628783 so all credit to the awesome people who figured this out. All I did was package it as a Magisk module!