Visual equivalents of the named colors listed in the dropdown may be viewed here. If a name does not appear in the dropdown, it is not supported because it will not look good with the default cover design.
A visualization of the total color space in the color names system is presented below. Light colors have been eliminated as the default white type is not readable against them.
Custom colors may be added via using the RGB form. Note that if provided an RGB value trumps the color nameselected in the dropdown.
It's understood that CMYK colors are the standard for print book production, but RGB is preferable for e-book covers. CMYK support may be added in future.
PANTONE (tm) colors of the year for 2000-2012 are provided for reference.
A huge list of popular colours with RGB values is available at colourlovers.com, including covers that have been tagged with the keyword "book cover".
The palet.ly site enables you to upload any image and create a color palette from it. Here is one I created.