It seems that some Nokia N8 and C7 AMOLED displays are defective. The most common complaint (there are several actually) is that the screen has a purple tint in darker areas. It's not clear yet whether this is a software or a hardware issue. It is speculated that Nokia uses two kinds of displays - although it's unlikely as all AMOLEDs are made by Samsung.

In the photo above you can see the problem (on the phone on the left - the clock and button shouldn't be red!). Nokia hasn't officially recognized this issue, but it seems that more and more users complain.