Slings and Arrows of Outrageous Fortune...
I realize that this is an old thread, but sitting here with two iPhone 4s in front of me-- one markedly yellow, one a cool light blue, there is a decided difference.
As another poster wrote, we didn't discover this due to visiting this web site-- or even, in fact, by putting the phones next to each other-- or beside my iPod Touch 2G that I've been using for years (and has a cool-blue screen, with whiter whites).
No, what happened was that one of the first mornings I used my 2nd iPhone 4, I wondered why the screen looked weird The background, supposed to be white in the news app, seemed yellow. Text was harder to read, it made the screen protector more visible. I fiddled with brightness level, but it still didn't look good. At that point, I thought, "What's going on here?" and grabbed my iPod Touch for a comparison. Shock of shocks! Whites were white on it, or perhaps cool-blue, whereas they were definitely yellowish on the iP4.
At very high brightness levels, or with a white silicone wrap around, the yellowish one looked OK. At any level, its photos looked better than on my IPT, and any app involving black backgrounds (night themes on ebook readers and astronomy apps) looked much better than on my IPT.
But reading at low-light and brightness levels remains the pits. I'm glad I'm not alone; that it's not my imagination. Alas, this IP4, purchased used, is no longer under Apple Care, so I'm stuck.
Well, unless the reports of a jailbreak, color-calibrator app are true. If so, that's clearly a reason I'll jailbreak for the first time.