Can you see the rainbow in horizontal bands as the underpainting of this landscape? Look for: Red in the foreground ( the bottom of the page), orange, yellow and green in the middle ground, and blue and violet in the background (the top of the page).
The girls have learned all about color and color relationships regarding feeling. Now, they are using color to show space. Warm colors (red, orange, and yellow) are aggressive and push forward; cool colors (green, blue, and violet) are recessive and fall back. Artists use this visual phenomenon to show space in their drawings. If you think about it... It takes a few tricks to turn a flat piece of paper into an illusion of deep space. Other tricks the girls applied are size relationships (making the objects in the foreground bigger and the objects in the background smaller) and converging diagonal lines (when paths, rivers, roads, etc. get narrower as they go back into space).
For their own landscapes, they painted a rainbow as the underpainting and drew on their lines (horizontal, vertical and diagonal). Then, they started filling in the shapes of heir composition with oil pastels. Next, they blended the oil pastels to make warmer and cooler colors, depending on where they are in the space.
The girls also mixed warms and cools to make neutrals (a fancy word for browns). And, they put opposite (or complementary colors) next to each other to create a visual "zing".
Here are some examples of the work in process: