Based on your description of an image featuring cool colors like greens and blues (along with a blue sky), the principle of design that seems most evident is harmony. Harmony refers to how well the elements of an image work together to create a cohesive look. The use of cool colors can create a soothing, unified effect that enhances the overall aesthetic of the landscape, contributing to a sense of balance and tranquility.
While radial balance may not be as applicable since radial balance involves elements radiating from a central point, and proportion typically refers to the relationship between elements in terms of size, harmony aligns well with your description of the cool color palette blending together in the landscape.