Visualizing color brushes in ellipses

@amrykid: ["...now I am wondering if we can present the colors in a prettier way..."](https://github.com/MahApps/MahApps.Metro/pull/989#issuecomment-34082814)

![image](https://f.cloud.github.com/assets/764517/2094048/1e886124-8ec4-11e3-89fa-9fbb241fd5ca.png)