It appears with 4 million units sold, Apple’s quality control mechanisms were unable to prevent a number of iPhone 4Ss with yellow tinted screens from slipping into the hands of consumers.
A number of messages have popped up in the Apple Support forum complaining of the issue, complaining of washed out screens with a yellow tint on their phones, especially when compared to the iPhone 4. While this could be simply an issue of different LCD’s being used between the 4 and 4S, it appears it is not, as only the black 4Ss appear to be affected.
Yellow tinting is an issue Apple users are familiar with. The phenomenon has affected Apple products from the 3G iPhone, iPad 2, and most notably Apple’s iMacs, which seem to be habitual offenders of the yellow tint. A possible explanation noted by a few people posting in the support forum is that the yellowing is a result of the glue used to put the screens together not fully hardening or curing. Many iPhone 4 users who experience the yellow tinting initially last year saw it disappear as the glue hardened.
However, the contrast issue could be a separate problem, with one user writing:
my 4S screen is less contrasty, and the whites are more yellow (beyond 'warm') compared to my iPhone 4 screen. The colors are less vibrant, and some are pretty washed out. I've also noticed that the screen is more directional than the 4 screen, and in some viewing angles it's more yellow, and in others it's more contrasty. I'm really hoping this is a glue issue, which could improve. I don't think I'll be able to get used to this.
Source: CNET, Apple Support