For anyone else that comes across this issue I have included the solution below. There is a great steel-wheels inventory example that is included with the community and enterprise edition that explains how to do this.
=IF( [PERCENTAGE_USED] > 50.0% ; "#339933" ;
IF( [PERCENTAGE_USED] > 45.0% ; "#00CC00" ;
IF( [PERCENTAGE_USED] > 40.0% ; "#33CC33" ;
IF( [PERCENTAGE_USED] > 35.0% ; "#66FF66" ;
IF( [PERCENTAGE_USED] > 30.0% ; "#99FF66" ;
IF( [PERCENTAGE_USED] > 25.0% ; "#CCFF66" ;
IF( [PERCENTAGE_USED] > 20.0% ; "#FFFF99" ;
IF( [PERCENTAGE_USED] > 15.0% ; "#FFCC99" ;
IF( [PERCENTAGE_USED] > 10.0% ; "#FFCC66" ;
IF( [PERCENTAGE_USED] > 5.0% ; "#FFCC00" ;
IF( [PERCENTAGE_USED] > 0.0% ; "#CD5C5C" ;
IF( ISNA([PERCENTAGE_USED]) ; "#CD5C5C" ))))))))))))