To be a good landscape artist, you have to be a student of nature. If you study the effect sunlight has on the trees, dirt, flowers, rivers, streams, oceans, snow, and everything else in sight, then you know the colors are in constant change with the time of day and the atmospheric conditions.
If you are painting your landscape from nature, you may find the hues changing as the sun crosses the sky. In many instances there is the need to finish the work in the studio. You should get in the habit of constantly making color swatches of what you see. If you do this, be certain to make good notes describing the time of day, the month (season), the atmospheric conditions at the time, and the exact color mixing formula. Always keep these filed and notated. It takes time, but you will eventually have hundreds of color swatches that gives you perfect reference material.
Here are some tips to consider when painting the sun into your landscape:
When the first light appears just before the sun rises, the landscape is flooded with shades of blue and purple. Apply these cool colors to your painting. The underpainting should be more of a lavender hue. This will create a comforting mood. However, to keep the painting from being too moody (as well as, keeping it more interesting), add a few touches of color, perhaps flowers or wild weeds or complement specks (tiny dabs of pigment of the opposite hue).
At sunrise, the temperature of the painting becomes warm and not cool. This is a completely different painting than the one started just a few minutes before when the first light was showing. The painting for this time of day should start with a Cadmium Yellow Light or Lemon Yellow wash. This color will dominate the painting and should be mixed with all the hues. However, as the sun continues to cross the sky, the cast of yellow becomes a deeper hue.
When the sun is directly overhead, a darker yellow should be dominant throughout the painting. Experiment with this but often yellow ochre and oranges will be a good place to start. Whatever mixture of colors selected, the value should be light because the sun washes out the darker tones. Trees, for example, should be lighter at the top than at the bottom. Be sure to notice that shadows are more harsh at this time of day.
As the sun falls into the western hemisphere, more oranges and reds and purples are evident. By sunset, the yellow hues have almost vanished. The closer to dusk, the cooler the temperature of the painting. These colors should be mixed into your foliage to make it appear as a natural sunset.
After sunset, your deep reds influence all the colors of nature. A Cadmium Red Deep should be mixed with all the hues in the painting. You need to experiment with this. What you are trying to do is tone down the green of the trees and the other foliage.
If the atmosphere has fog present, then a painting needs to accurately show this. Amidst the painted fog, an atmosphere of softness must prevail. With a mixture of white and the complement of the colors used to paint the scene that must be fogged over, use a dry-brush technique by overlapping strokes in different directions until the edges of the area are softened. This should be done very gently. If not satisfied with the results, wait until after the paint dries and repeat the process.