Cool colors are made with blue, green, purple, or some combination of these. Cool colors might make you think of cool and peaceful things, like winter skies and still ponds.
In this painting by Georgia O'Keeffe, White Barn, the cool blue of the background contributes to the quiet feeling. The simple shapes and stillness of the barn give this painting a restful look, and the blue makes it feel even more peaceful. Imagine how different the painting would look with a bright red sky—it might seem more exciting than restful.