I think its a OS problem. But there is a fix!
I had my phone replaced for the same issue... weirdly enough, the new iphone they gave me worked fine for a couple of minutes, but then after restoring from iCloud it went back to yellow...
Go to Settings / General / Accessibility / Display Acommodations / Color Filters
Then you can adjust the color of the screen... I had no clue this setting existed but it was able to to turn "Color Filters ON" then tap "Color Tint" and found the right color for me... more blu-ish/whit ish if you go with intensity all the way to the left and hue around 60% to the right.
I still think this is a setting bug witht he OS because I compared my phone many times with the ones at the apple store and they all seemed white vs mine that was yellow..... the problem was that I never saw those phone's settings before changing mine... next time im near the Apple store I'll take a closer look to the color filter settings.