What are Harmonious Colors?
Four or five colors that are close to each other on the color wheel are harmonious. On this color wheel we have chosen four colors that are harmonious: blue, blue-green, green and yellow-green. They all have blue as a common primary color in them.
We could add blue violet into the mix and still be harmonious and maintaining blue as the common color with a touch of red from the blue-violet.
We could choose to add the yellow instead, and they would have the two primary colors blue and some added yellow.