Almost each theme uses one, two (or more) recurring colors.
They are usually used for each backgroung, or each link, or each even and odd table's row and a lot more uses.
IMO for each theme it should be mandatory the definition (via setting page or embedding them) of two (or more) different theme's color.
Then, at each needs, even the core of moodle may use them. This idea well fit the need of MDL-31712, too.
Hoping this may be of help for someone.