Many books have also been written on this subject, so I will just put my two cents in for what I consider a simple solution for making colors harmonize in landscape paintings. Use an analogous color scheme. This means keep your main colors close together where they appear on the color clock, only using opposite colors for graying and cooling. The best policy is to make sure as many colors as possible in your artwork share the same common mother hue. This applies to everything where color is involved, even interior decorating in which the walls, rugs and furniture have similar colors. When doing autumn scenes, avoid the red hue and green hue as isolated colors. They don’t share enough of the common yellow-orange denominator. I tend to use green just as a spice in autumn scenes in small amounts, leaving the yellow-orange, orange and red-orange as the predominant hues.
In this color scheme, I eliminated yellow because it rarely appears in landscapes. Yellow Ochre Pale and/or Burnt Sienna can be added to green to make it more agreeable with the other pigments. All the colors will share yellow-orange. For over-the-counter pigments, a working palette for autumn foliage can be Yellow Ochre Pale, Cadmium Orange, Burnt Sienna, Permanent Rose and Viridian.
Spice up your color mixtures with touches of red. With the exception of the mountain, most of the areas share a yellow-orange hue. There was a risk that the evergreen trees ended up being too green. To offset this, adding Burnt Sienna that contains the influence of the red-orange hue pulled them together with the yellow-orange trees. To keep them from looking isolated or out of place in autumn scenes, I tend to avoid showing any summer green colors that would not contain some red mixed in. They would at least be similar to a martini olive.
Another way to establish color harmony is by repeating surrounding colors all around your painting. Snow, waterfalls and white walls will sing if you echo colors in a subtle way that appear elsewhere. This concept is known as reflected light.
In real life, when you look into cast shadows on snow, rocks, waterfalls and white water, they will take on a gray-blue appearance. You won’t see the echoed colors. Nature has its own way of harmonizing color. With pigment, we don’t have that luxury so we have to go the extra mile to end up with color unity. This means we will need to borrow colors from different areas of the painting, add them to other areas, and find pretexts to repeat these colors. In this painting, the cast shadow is picking the pinks from the flowers and the orange from the walls. The sky is the primary source of influential color. This is another example of why you can’t paint just what you see in nature. Not only are we to echo colors in shadows, we must also weave surrounding colors into main areas. There are touches of green in the pink wall on the right. You can justify this green addition as mold on an old wall.