Background colors

How do I enable my users to change the background color if I have an image for my background? For example, a tiled, repeating background jpeg that is almost all white.  I don't want to use a method that affects any characteristics besides background color (for example, turning off All graphics to get rid of the background).

Thanks for your help,

Benjamin J. Simpson
Education Associate, Web Development Group
NASA Ames Research Center
eml:bsimpson@mail.arc.nasa.gov
tele: (650) 604-3292