This is a known "problem", most likely because different display panel manufacturers used by sony give different white balance setting.
Here in indonesia we can confirm that different unit of zu can have different display's default temperature - not just comparing it with other brands. Range from very white (in other thread here in xda someone said her display is almost bluish), warm, to very noticable yellowish type. I heard that Z has the same problem, which is solved after sony release firmware with white balance setting.
I can say that my zu is very white, and probably tilted to a little bit warmish (Because I'm certain it's not bluish) especially with auto-brightness on. And I like it.
If your display is distractingly warm, or even distinctively yellowish, claim warranty. Find a friend who own another zu w/ comparably normal display to show that your is a defect. Or if there's a display unit available maybe you can use it as comparation.
Anyway, good luck.
Sent from my C6802 using xda app-developers app