Ok have some more thoughts about your problem.
It seems to me, (despite the cloudy weather on the pic), that the sun is somewhere in front of a camera. This would be a difficuilt light conditions. You've mentioned that the modes were P and Auto, which means you let the camera decide what's the best, wich is not a good idea here. (besides Auto and P are not much different)
For a beginning try to take the same picture when the sun is in a different position, just to see the difference.
Second you can get a better results using Tv and Av modes it requires some knowledge but generally it's not hard to figutre it out.
And third use exposure compensation (I've mentioned that already). After all it was made for such an occassion.
Finally try to switch light mettering mode to Center-Weighted Averaging. (read p.83 of your manual). That might help better than just switching shooting mode.
I've noticed that sometimes I have different greens on pictures taken seconds apart, but generally this problem is about exposure.
Camera just gets fooled around by difficuilt light conditions, so that's where your eperience as a photographer kicks in 
Hope that helps.
P.S Don't mean to be rude or disregard anyones opinion over here, just sharing my thoughts about the problem.