For example, during the fall, one might often see the changing leaves form an analogous sort of color scheme, progressively moving through the color wheel to create a gradient in its natural pattern.
High-key color schemes have a lighter value, having white added to them or water in the case of watercolors.
Having a high-key analogous color scheme can give a piece a stimulating shimmer that pleases the eye, making everything seem the same color at first until approach.
The colors are pure and aren't affected by their complements which grab attention.
This was commonly used in impressionism by artists such as Monet, Pissarro, and Degas.