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.