Historically (since about the iPhone 4), the yellowing has been there from brand-new models. In most instances, the colour tends to sort itself out within about a week. Though there's no
definitive explanation as to the cause of the issue, the hypothesis about glue seems to be the general consensus. Regardless, the yellowed screen is nothing new and nothing to worry about.
Based on this, the floor models will be a lot less yellow due to how much use they've had. Also a lot of floor models will have been used practically from launch date, so it's got a solid few weeks on your one.
Of course I'd always recommend speaking to Apple if you have any concerns. The earlier you report it, the better; so if you do need to swap it down the line, you'll have that case history available. The absolute worst-case scenario is that the yellowing stays and Apple replace your device, so one way or another, it'll be sorted.
I hope this provides some reassurance.