It's basically an advanced way for the screen to properly adjust both colors and white balance based on ambient light conditions. If having an extremely accurate white balance and color gamut is important to you, then the True Tone display is really the only way to go. It's not like Night Shift, i.e. there's not a noticeable yellow tint.
http://www.displaymate.com/iPad_Pro9_ShootOut_1.htm
Thanks.
That link suggests it "The idea is to make the display behave more like paper reflecting ambient light and taking on its color and brightness." So actually, the colour reproduction is LESS accurate with True Tone, as it is attempting to adjust according to ambient light, like a reflective surface, such as paper. Whereas the goal with colour screens in TVs, monitors, etc, especially for photo work, is colour accuracy regardless of the environment.
So it sounds like True Tone is good at making iPads usable and pleasurable, but at the expense of colour accuracy. Is that fair?