To get the RGB values from a pixel of the screen with coordinates (x,y) in Python, I do:
import PIL.ImageGrab
rgb = PIL.ImageGrab.grab().loa
If you do:
import PIL.ImageGrab
PIL.ImageGrab.grab().size
You will see the resolution that python is detecting in your system. If you are using Windows, you have to go to Screen Configuration and then change: text, aplications and another elements size to 100%.
If your resolution is 1920x1080 ,as you said, this should fix the problem.