Betty Edwards writes in her book, Color: "Everything about color has to do with relationships, and the questions you need to ask in order to understand color are all about relationships."
What you see at right is a color wheel. If you start with the three primary colors, and then add the secondary colors (giving you six colors), and then add the color that is a mixture of the adjacent primary and secondary colors, you then have twelve colors.
What combinations of colors please the eye? Studies have found that opposites do. For example, purple and yellow, or red and green. To show that this is true, several people have complimented my current painting, a still life of a red pepper and a sliced red onion. The red pepper is a bright red with a green stem. The onion is purplish with yellow highlights. This was quite by accident on my part - I was simply painting what I saw. But as I step back from it, I do see that aspect.
Artists also find that triads - creating a color and then using the fourth from it and then the fourth again - creates a harmonious mix. Red, yellow, and blue are one example. Orange, purple, and green would be another.
I doubt that it has anything to do with this, but music also has twelve notes between octaves. To create a harmony, you use not the opposite or triads, but rather fifths or sevenths. But there's a rhyme and a reason to understand harmony in both color and music.
So... I wonder how this idea of hue (a color from the color wheel), value (light or dark), and intensity (bright or dull) transfers to harmony in friendships and marriages. I'm too tired now to consider it more, but I'll be thinking of it...
ETC: More in the comments...