The yellowness is not a brightness or dimming problem. It is a color temperature issue with the backlight bulb.
I have had four TPs now, and two right in front of me. I've seen three different shades of color temp backlight.
I have also had three Diamonds, and each had a different color temp backlight.
They range from very yellow to slightly yellow to one TP that was very white (higher color temp).
I compared these phones on the same screen (Programs or Settings menu, white background) with brightness manually set. Compared to my Centro, they are all very yellow. The Centro's LCD color temp is much higher.
I'm guessing it is either a quality control issue or different suppliers. Manufacturing date did not seem to matter, it's a toss up.
I made this post some time ago and includes some comments and pictures in the thread (look for links):
http://forum.ppcgeeks.com/showthread.php?t=41533
Edit: BTW, each of these devices were purchased new, not replacements.