I had the same problem with two brand new 256gb 8+s. All the settings were good (no true tone, etc) and they were quite yellow and noticeably dimmer compared to every display I put them against (Xs, 8+s, 8s). Even the screen on my old 6 looked better all around. Like, significantly better.
I'm wondering if there was a bad batch (maybe a large one), because the screens were just unacceptable to me for a near 1k device. Everyone at the Apple store agreed with me about the yellows and the brightness, but unfortunately I purchased through my carrier and I wasn't about to get a 2 day old device sent for a screen repair, so I returned it, got another one, and same problem.
It was a huge hassle and finally I just got a regular 8 and let me tell you, this display is worlds better than the two 8+s I had. The effect that true tone has on this display vs the 8+ is markedly different too. Even in dim lighting it is less yellow than the 8+ with true tone off.
Very disappointed to get two in a row like that. I almost went to Android, because at first I got an X but suffered horribly with eye strain and migraines due to (presumably), the PWM mechanism on the device. Also true tone is completely broken on the Xs I have tried it on (over 10 at this point).
Happy with my 8 now though. Good luck!