Which harmony includes one color plus the two colors on each side of its direct complement?

The correct choice is split complementary harmony. This harmony specifically comprises one main color and the two colors found adjacent to its direct complement on the color wheel. For example, if blue is the main color, its direct complement is orange. The split complementary scheme would include blue alongside the colors on either side of orange, which are red-orange and yellow-orange.

This approach creates a vibrant color palette that emphasizes the main color while adding contrast through the use of the two adjacent hues. It allows for a balanced yet dynamic visual effect, making it a popular choice in floral design and other artistic applications.

Other types of harmonies like triadic harmony involve three colors evenly spaced on the color wheel, creating a different dynamic. Double complementary harmony would involve selecting two complementary color pairs, which introduces even more colors that don't follow the one color plus two adjacency rule. Analogous harmony consists of colors next to each other on the color wheel, lacking the complementary component entirely. Therefore, the unique structure of split complementary harmony makes it distinct and effective for achieving a certain aesthetic in design.