Colors that are created by mixing together many colors tend to be richer. I like to think of them as being “rich” because they have been created with many colors instead of just two. For example, we could make pink by adding white to red. We could also add a tiny amount of green, purple or any other color we choose in addition to white and we would end up with a pink that is considered “richer” than one made with just white and red.
A color that is richer tends to relate better, or be in harmony with other colors in your art simply because that color has been created with a small amount of all those other colors.