There is nothing worse than painting outside in the middle of the summer when everything is green, unless you're painting in the middle of the summer during the middle of the day when everything is flat and green. One option is to go home and take a nap until the late afternoon light gets more dramatic, but the best option is to look for value and color changes. Even though the sun is overhead, there is still a direction where you are looking into the sun which gives you back lighting and more shadows. Also use the planes of the landscape to give up more dark and liht values. The four planes of the landscape: 1. Sky Plane 2. Ground Plane 3. Slanted Plane 4. Upright Plane. These four planes are at different angles so they will all be a different value. The sky plane will be the lightest, the ground plane the next lightest, then the slanted planes, and then the darkest will be the upright planes. If you push the differences between these planes you will get more contrast.
When the summer colors are all green look for variations of green. In the painting here I see the ground as a yellow green, the trees are a darker orange green, the mountains are a grayer green and the distant mountains are a blue green. So by leliberately pushing these color differences in the green, I end up with much more color variety.