By default, True Tone is enabled in iPhone. If you are not a fan, then you can disable it.
Disable True Tone in iPhone
Step 1: Open Settings.
Step 2: Hit Display & Brightness.
Step 3: Now, you can see True Tone. Disable the toggle switch next to it.
Note: To revert, follow the above steps but instead of disabling, enable.
Open Control Center.
Long press the Brightness slider.
The first icon from the right side is for True Tone. Tapping it will turn it on or off.