As in the past, let your screen burn in for a few days. This typically goes away and is not unusual for the first phones made. It's typically the glue settling in after the screen is used for the first time. If after a week or so, the screen is still not right, then make an appointment with the Genius Bar at your Apple Store, but I wouldn't be concerned with one of the first units delivered in the world.
The same thing had been reported for the iPhone 3G, GS, 4 & 4S when first released and "fresh" from the factory. It's probably a coating in the manufacturing process that evaportes over a short time. Also, the color temperature of LCD screens change over time - there will always be differences.