I used a brush set to darken, at opacities ranging from 25% to 45%. First I selected a sky color and brushed the sky above the trees. Then I selected a yellow from the trees and brushed them. For the trees, I decided to use a duplicate layer, and set the layer's opacity to around 60%; that's because the brushwork was blurring the shape of the leaves.
To get this right, it really has to be done on the original image, not on a 72-ppi copy. But I wanted to show the idea of what I had in mind.
Dan