Every iPhone that I owned (several replacements over the years) had a yellow or darker tint on one side of its screen. My iPad was just like yours and now it is perfect, I dunno how but the tint has gone after a few months.
My iPhone 6 has a yellow tint on the bottom and I'm keeping it. I m confident that it may improve with time and that most replacements are going to be same or worse.
You said the picture is in full brightness and in a dark room. I bet it's not bad with light sensor on, or normal backlight levels. Is it? If I were you I d wait 10 days.