Park Stitching - Unfixable colour shift?

I have shot the images of the park and begun to stitch them together. By using layer masks and a bit of warping I've been able to merge the first ~35% relatively well. 

However, once moving around to the 40% - 60% mark a colour shift has taken place, and I've been using levels and curves to try and sort out the stitch, which is working, a little. The trees and grass were quite easy to stitch here however, which I thought would be difficult. The bark chips are a pain, and by stitching them in a curved line, instead of a vertical one, should make the stitch look less obvious. However, Once hitting around the 70% mark there is a crazy colour shift that, after using colour match, levels, curves, and brightness I have been able to reduce to the image below.

Compared to how it was beforehand, this isn't too bad (and that's saying something) but it's still bad enough to make the image look bad.