I\'ve set up a simple jQueryUI progressbar:
        
One simple thing would be when you are initializing the progress bar with values in your js you do :
$(progressBarId).children().css('backgroud',) ;
Since you want different colors for different progressbars, you can do :
if($(progressBarId).value() <10 )
//set a color
if (..)
//set another color
I hope this answers your question. I tried doing what the guy has said in the first answer, but could not get it to work so tried this and it started working.