Why are the outside walls of my house different shades?
Solution 1:
At looking at your pictures, i have an inkling that your walls are opposite when it comes to day and night. I think its a slight glitch, somehow the second floor walls think its night when its actually day, and think its day when its actually night.
they are very simular, i think
if you disagree, then i guess it could be a contrast issue, or what raven said, a brightness issue, try colouring the wall yourself, play around with its brightness and contrast (if your able to)
hope any of this helps Rob