Color variations withing simple value shapes:

A few extra high key shapes make all the difference

before the little high key shapes were added to the dark mass on the left

Before the small high key negative shapes were added between the tree trunk in the dark shape on the left, the eye was abruptly stopped and at the point where the light value, mid distance strip met the first foreground tree trunk on the left.  After adding the small negative shapes between the trees, the eye moves back and forth between the mass on the left and the distant mass of trees, taking in the subtle changes of color within the masses.