Help with a background image

Hi all,

I wonder if someone can help me with the following issue.

If you view my image on the following link you will see that in the top left hand corner there is a square box which is a different shade of green to the rest of the header.

I need to rectify this and blend it in with the rest on the header colour; however, I do not know how to do this and would appreciate some assistance in doing this.

I'm a noobie to photoshop so any help is much appreciated.

I would try a combination of filters but would try curves first or perhaps copy paste a piece of the other side to mach