How to Choose Garden Color Schemes
The photo shows and example where a color-contrast was employed... Color theory tells us that juxtaposing such colors provides contrast. But you don't have to rely on color theory for this conclusion.
Use warm and cool colors smartly. Warm colors are red, orange, yellow, purple, they represent energy and life; whereas cool colors are white, pink, blue, green, grey; they represent calmness and peace. Use them according to the mood you want to create in your garden.