This first image is the very first layer of underpainting.

So far so good. This painting is going to be as much about the clouds as it is about the marsh and I went in to add more detail and depth to the sky, while also adding detail and darks to the ground to balance.

I'm not unhappy with the results, but I can't help wondering if I might like it more with darker tones near the tree line and in the clouds. The surface is not dry enough to add a wash to it so instead I take a digital image and add the changes with Photoshop.

The changes are subtle, but it gives me the information I need and soothes my impatience at the same time.
I use this method frequently, finding that it's especially helpful and time saving on larger work.