Exchange it because you are used to the color hue on your previous iPhone? That doesn't sound like a defect, that sounds more like OCD but, hey, if the Apple store is willing to keep sending you replacement after replacement until you get the screen that you PERSONALLY like best, then more power to you.
I was under the impression that they would not replace it unless it was a defect (i.e yellow spots, dead pixels etc). A slightly warmer hue doesn't qualify as a defect in my book; just the characteristic. Like I said, more power to ya. I am often amazed at how many god damn replacement iPhones anal-retentive people manage to get from the Apple stores....
http://www.youtube.com/watch?v=YFHypT5gnbw
go to 3:20 i think this is a case by case basis. The iPhone 4S in this video looks way sharper than the iPhone 4 screen. The 4S has a much better contrast and dynamic range with deeper blacks.