This. A lot of people do not understand (relatively young) OLED technology. OLED screens ALWAYS display a warmer/yellow tint, especially from Apple. The reasons why are simple:
1. Apple highly specializes in color accuracy reproduction and calibrations. The new iPhone(s) that utilize OLED technology are calibrated to the truest color imaginable with the technology at hand. They do this before they are packed in boxes and shipped/sent off.
2. In terms of professional digital color reproduction and calibration, white isn't necessarily white. A well calibrated and color accurate monitor/screen will ALWAYS display a hint of yellow and will give off a warm color. Those who aren't used to or trained on high quality, calibrated screens will notice the yellow/warmth right away and their eyes as well as their minds will reject it. This is normal.
We human beings favor the blue end of the light/color spectrum because it seems brighter and more neutral to our eyes. Just because a digital screen looks brighter and bluer (which is perceived natural to us) doesn't mean its overall correct in reality.
3. The OLED technology in modern day mobile devices is still young. The Note and Pixel lines all have been plagued with the same "issues"/complaints. Screen burn in, deeper than normal yellow tint, lack of uniformity and the like. Its because this technology is bleeding edge and takes years of RnD, and fixes to get them to where, in our (literal) eyes as the consumer, to satisfactory levels. Once again, even if the screen is absolutely perfect, it will still seem "yellow ish" to the average consumer.
4. I encourage everyone to go to their local electronics store (Best Buy, Apple, Fry's, Microcenter, etc), look for their devices that have OLED screens. Compare those devices to your non-OLED screens and your iPhone X/XS, Samsung Note 9, 9+ and Google Pixels. You'll notice that the yellow/reddish tint will still be there. The intensity varies between phone manufacturers and the screens themselves.
5. One last important thing to note: Your screen will "cool down" over time. Your eyes and mind will adjust to it and you will get used to it. Give it time, you WILL adapt to it. I personally can NOT go back to turning true tone off and/or use a screen/monitor that isn't professionally calibrated. Hell, even my work monitors and laptop screens are calibrated to the best of my ability and the best of the ability of the technologies within each device.
If you can't wait that long and are still miffed about the color after some time, adjust the color to your liking. You can either turn off true tone or go to Settings > General > Accessibility > Display Accommodations > Color Filters and adjust the color of your screen by yourself (self calibration). If all else fails and if you still aren't satisfied, simply return your iPhone before the 14 day mark. You shouldn't force yourself to use a $1000+ phone if you aren't satisfied with it.
To those who are experiencing MAJOR color uniformity and shift issues, please take it to your nearest apple store and have someone at the genius bar take a look at it and compare it to the iPhone(s) at their store. You may or may not need to get it replaced. A $1000+ phone should NOT have a bad screen. That is just it.