Some iPhone 4S users are reporting their screen has a yellowish tint. The tinting problem supposedly affects the whole display and gives everything a washed out appearance when compared to the iPhone 4. This is apparently different from the yellow tinting problem of the iPhone 4 which appeared as yellow blotches on the screen. The iPhone 4's problem was caused by glue used in the manufacturing process and went away once the glue dried.
Customers on Apple support forums are actively discussing the current issue, which appears to affect the black models more than the white; though some white models still have it. Many suggest this is the result of using more than one screen manufacturer. One of the manufacturers may be producing a lower quality display.
This is not the first time an Apple product has been plagued with display issues. Late 2009 iMac models reportedly suffered from yellow screen tint and flickering issues. Earlier this year, iPad 2 supply was supposedly constrained due to a light bleed issue caused by their LG-manufactured displays.