On Mac OS X, the floating syetm color palette is accessible from the "Personalized..." option when choosing a Fill Color for Background:


The floating system color palette has the ability to show up to 10 rows of custom colors:


By default, this window only exposes 2 rows.

In order to expand the window you must locate the dot that is below the rows of custom colors in the color palette:


Then you can click and drag the dot down or up to expose or hide additional color bubbles.