I have just installed Ubuntu Touch on a Samsung Galaxy Nexus phone (supposedly an unused one).
The problem is:
The middle part of the screen displays a color tone that's more yellowish than the parts where the (previous) black Android bars were displayed (at the top and at the bottom of the screen) before my Ubuntu Touch installation.
Could it be that there is some sort of Android interference on the technical level (see https://wiki.ubuntu.com/Touch/FAQ#How_is_Ubuntu_Touch_connected_to_Android.3F) that causes this or does it mean that the device is not unused and that Android was indeed used on it long enough for the middle part of the screen to become more yellowish than the surface of the top and bottom bars where the screen would remain "more unused" (and therefor less yellowish) because the bars were black?
You can even see Android's arrow pointing up, at the center, in the 1st pic: