MP6 has some awesome color schemes, but maybe you want to add more. Here’s how you can add your own in a plugin.
Edit (11/19): As MP6 was an experimental plugin, most of the code here will not apply to what’s been merged into WP 3.8. The idea is the same, though (except the customizer colors were dropped).
Note: These instructions are for a branch of MP6 which is currently not up-to-date with the rest of MP6. MP6 is now using SASS, and while the theory is the same, details listed here are not accurate. Check out this post for a more up-to-date method of working with MP6.
package.json and Gruntfile.js; so you can build new themes using grunt.