True Tone is an automatic adjustment for your display’s calibration.
If you don’t use it, then you will be getting whatever Apple’s default is for your display. You can fix this by manually creating a custom calibration profile. There are some (pretty expensive) hardware/software systems for this, or you can use a basic one built-in to macOS:
- Go to System Preferences → Displays → Color
- Click Calibrate… for a basic calibrator. Hold down Option when clicking it to get a check-box to enable expert mode.
- Follow the instructions to create and save a new display profile
Once this is done, you can select your profile in the Displays preference panel. If you have multiple displays, create a separate profile for each one.
You may want to do this even when you are using True Tone, because it will use your profile as the baseline for its adjustments.