Your screen is defective, it's not the computer itself. Like anirban said, it's like a "crack" in your screen, a "print screen" won't show it.
The Computer just send "image information" to the screen and that's the screen itself that does not display it correctly. When you do a Screenshot with the computer, the computer just "save" the information it sent to the screen but there is absolutely no way the computer can know HOW the screen actually display the image, so there is absolutely no way a "screenshot" (or Print Screen) can show how exactly the image is displayed on the screen.
The only way is to take an actual camera and take a picture of the screen itself.
About your issue, give us some details :
-Is it a 15'' inch or a 17'' inch ?
-What do you mean by the day they came out ? In June 2007 when they updated Macbook Pro or in 2006 when they actually launched the Macbook Pro line of notebook ?
-What do you mean by "dull"? The yellowing issue is only affecting 1/3 bottom of the screen. When you do the test you don't even need another computer to make a comparison. The bottom white square will be darker and slightly yellow compared to the one on the top. IMPORTANT : Any color difference that is affecting all the screen, like your color all seems washed out compared to others MBP, is only a matter of monitor calibration in the display setting of MAC OS X. If it is the whole screen who seem "dull", it is not a defect.