One of the first psychologists to apply coloring as a relaxation technique was Carl G. Jüng in the early 20th century. He did this through mandalas: circular designs with concentric shapes similar to the Gothic churches’ rose windows. They have their origins in India.

"When coloring, we activate different areas of our two cerebral hemispheres", says psychologist Gloria Martínez Ayala. “The action involves both logic, by which we color forms, and creativity, when mixing and matching colors. This incorporates the areas of the cerebral cortex involved in vision and fine motor skills [coordination necessary to make small, precise movements]. The relaxation that it provides lowers the activity of the amygdala, a basic part of our brain involved in controlling emotion that is affected by stress.” (cited from The Huffington Post)