I have this same issue. On a white background it looks tan, orangish (I am color blind) but I can definitely see a spot, smudge, streak. I have it on left side of the screen, across the top, and on the right side of the screen. I took it to the Apple Store and they said this was normal behavior. I bought my iPhone on 3/24 and had it replaced once already because my first iPhone locked up 2 to 3 times a day. This second one has been rock solid, but now I have this annoyance. The only thing different is that I have been flying a lot lately, I wonder if the pressure changes are causing a fault...
Anyways, I asked for a replacement, because I deemed it to be a defect, but the Genius wouldn't give in, said that it has to get worse, well tonight it got worse. Also said that it might resolve itself. I am concerned because these units only have a 90 day warranty... BAD APPLE should be 1 YEAR for manufacture defects... Every other piece of consumer electronics has this...
Anyways, I'll take my iPhone in next Friday or Saturday and show that it has gotten worse and hopefully get a replacement. Then, I'll be the Apple Care because I won't trust my replacement to last more than a couple months.