I'm guessing it will retain the warmer white balance when true tone is off, much like the 7. This was done for preference by apple (idk why), not just because. That does mean that with true tone on ofc, it will still get quite yellow, which is the feature.
You can only change the color temperature (aside from adjusting night shift - which only makes it more orange, not less) by jailbreaking.
My iPhone 8 is being delivered tomorrow though, so I will let you know how it compares.