From the jQuery API ( http://api.jquery.com/animate/ ):
All animated properties must be numeric (except as noted below); properties that are not numeric cannot be animated using the core jQuery functionality. (For example, width, height, or left can be animated , but the background color cannot be .) Property values ββare treated as a series of pixels unless otherwise specified. em and% units may be indicated where applicable.
, jQuery.color: http://plugins.jquery.com/project/color