I have 3 X's in my family. They are all yellow. My friend told me his is yellow too, and we both agree that the 7 Plus screen is better (brighter, whiter, more pleasant).
I really don't understand people raving over the new OLED panel. It just looks like a step back compared to the 7 Plus in every way. It's also dimmer.
However, if I give Apple the benefit of the doubt, this is yet again, a result of the panel lottery and early production. Last year, I distinctly recall my first 3 7 plus sets having horribly yellow displays (when I showed the Apple employee, she said 'oh that's night shift' And went in to settings, and issued my refund when she saw it was off). I got rid of those, and waited until November to get a sim-free phone. Those phones came very white. It was hard to believe I had bought the same phone. And it was obvious that all the people claiming Apple had "purposely calibrated a warmer profile" were absolutely wrong. I had to exchange that unit, and again, the newer set came on the whiter side, although not as white as that first set I had.
This leads to this important note: ALL the panels will come with some tone variation. It is just the result of panel variability. While all my X panels are yellow, there are very minor subtleties to how yellow they are, and if there are blotches or if they are uniform.
Now, whether X panels will get whiter later in production, or there are already whiter panels out there, I don't know. I know there is one panel out of my 3 that I am considering keeping for myself (since my other family members don't notice this sort of thing), and the other two I can't really live with.
This will go on so long as Apple doesn't pressure the upstream component suppliers/manufacturers to get it right. And since most people would never notice, it will likely not happen. The only solution is to either deal with it, or exchange it. And exchanges can go on and on.
*other important side note: OLED panels have a habit of turning bluer over time. But not in a good way. I remember my Galaxy Nexus OLED panel was blue after 2 years of use, and did not start that way. So while I normally say things won't "improve" over time for screen yellowness, for OLED panels there may be some color change. Whether or not it will turn out well I can't say.