Very tidy image Mike, but it struck me that the background sky has some subtle colour variations.
I dropped the image in to CS3/Photoshop and sure enough...if you have a look at the individual channels in (*native* no stretching... honest
), there is some mottling in the red and blue channels yet the green looks perfect.
As for a cause/fix....I'll leave it to you ...you may not want to bother
Peter