Working From Photographs
Contemporary artists rarely rely on photographs to inspire their art. They frequently eschew such sources, considering photographic references to even be a distraction, as their primary concerns are for artistic elements such as shapes, colors and focal points, and don't wish to be confined by physical reality. Creators of representational art, however, are inspired by reality. In fact, the more realistic an artist wishes to be, the more important such references become. The tiniest bits of detail can bring a painting to life. Faking it, unless you have a lot of experience with the subject, simply doesn't work.
It helps to start with the right photograph, and the first thing I look for when referencing a photograph is the light. The light on this particular image was spectacular. Interpreting it in a painting, however, required some planning.

First things first, of course, and that's the sky. I always build my paintings from background to foreground, because I will be layering one segment over the other and in this case, I want the sky to peek through the branches and limbs laid over it. Don't worry so much about the perfect blended appearance. There will be a lot of detail in the foreground, covering up your mistakes. Just be sure to thoroughly cover your panel and avoid the appearance of streaks.

The first under painting. Very loose and casual. This is more to help define the darks and lights as shown in the source photo. There will be a lot of layers of detail laid over it, so again, don't go for perfect.

The first layer of trees. I rely on the lighter colored gold/green to contrast with the darker red/greens to begin planning the light, which is coming from the right side of the scene. Here's a close-up of the concept:

Gold is not yellow. Rather, it is a blend of strong yellow with a little red and a touch of blue or green. For colors of the shadow areas, I start with a dark green and add touches of dark red. Depending on the colors you choose, you can get anything from a reddish green to nearly black. The important thing here is to use the dark colors to begin defining the shadow areas.

By now, you can see the development of the light. From this point on, it will become a matter of where and how to place the detail. You will begin mixing your dark greens with just a touch of white to create the right green to lay over the darks and begin defining the branches and limbs. More white creates lighter, opaque greens; less creates darker tones.
Be careful not to go too fast. Go in stages. Don't be in a hurry, you will always have time to lighten or darken any area, according to your whims.

Here I have decided that the foreground foliage needed more bright gold/green for more highlight effects. Also, by covering the background trees with lighter grey-green they are beginning to get some atmospheric perspective and allowing the foreground trees to come forward.

More darks, then more lights.
Finally the foreground.

Add a little better shadow patterns in the hillside, and some flowers for color, and take a break.

One of the beautiful things about egg tempera is that you can always lay light colors over darker ones to emphasize detail or brighten some portion of your painting. For more information on adding light to foliage, please refer to my earlier tutorial, "Begin to Paint".