Why my Galaxy Nexus display is unable to present solid colors?
It's likely the matrix which you are seeing.

http://en.wikipedia.org/wiki/PenTile_matrix_family

The Galaxy Nexus uses the RGBG matrix just like previous galaxy, however it's much higher resolution than many other pentile displays.

Here's a magnification of another, similar screen from a different samsung phone. http://www.droid-life.com/2011/09/28/samsungs-super-amoled-hd-screens-bring-back-the-pentile-matrix/

If that's not what you're seeing then it may be a hardware issue.