Hello and thank you for contacting the Apple Support Community! I'd be more than happy to help you out today:)
First thing's first, check to see if Night Shift is on for your iPhone with the yellowy display. You can check this by going into Settings, then Display and Brightness, then Night Shift. If Night Shift is on, turn it off. If the screen is still yellowy, contact Apple Support through this link: https://getsupport.apple.com/
From there, you can open a support ticket and live chat with a representative, book an appointment at your nearest Apple Store, or call Apple for over the phone help.
I hope that helps and I hope you have a wonderful day!