A sensor embedded near the earpiece measures the color temperature of your surroundings and adjusts the white balance of your screen accordingly. If the lighting around you is too yellow, your screen will usually appear too bluish and thus color accuracy drops. True Tone compensates for this by warming up the white balance of your screen to make the whites whiter again. I’ve never found it too yellow, you might have been used to a screen without true tone and thus your eyes are still adjusting to this new feature. I’ve been using it on my iPad Pro for months now and I find it awesome. Otherwise, check to make sure your night shift mode isn’t actually on.
EDIT: Typo