Interesting results with my Datacolor Spyder X colorimiter. Also threw the iPhone 5s in there for the lulz since it was on my desk. Turned off all auto settings (night shift, true tone, auto brightness) and set them to full brightness. Used the wide-gamut LED calibration profile for my probe. Loaded an about:blank page into Safari to get white.
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While I'm not sure if the "glue dry" claims are true, it could be possible that some wear-in is necessary for it to even out. OLED screens are notoriously non-linear and difficult to calibrate. That and different glass coatings can cause different types of viewing angle shift. It seems out-of-box yellowness is common for many even going back to previous phones, but not sure if any resolved on their own or not? If I don't end up exchanging it maybe I'll try probing my 11 Pro again in a week, and after iOS 13.1 releases. Could also be possible the screens are running in the wrong colorspace or mode due to iOS 13 bug? Perhaps this has to do with tweaked HDR / SDR handling or tonemapping. The standard UI runs in SDR space. We'll see.