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you can't. i believe it's because the characteristic of the lcd.

the first batch of iPhone 4 have the cooler tint like mine when i got it a year ago, my wife's iPhone 4 which i bought for her 5 months ago have that yellowish tint.

i prefer the yellowish though because it's easier on eyes when reading websites with white background.

that's why iBooks app has a sephia mode because i think it's easier on eyes.
 
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