Conditional Formatting in Excel

I have built a GANTT chart using Excel, and have been able to create the formulae in the task bar cells that allows the proper display of those bars in the chart body - with the correct start date and end date for each task.
What I now want to do is employ "conditional formatting in a cell based upon content in another cell". The chart has a column - the cells in which contain an arbitrary "task completion percentage" and I want the cells in each task bar to reflect a different color based upon task completion (i.e. 100% - dark green; 75%-99% = light green; 25% - 74% = orange; 0% - 24% = red).
What I want to do is have each task bar cell read the appropriate completion column cell and apply the appropriate color - but I cannot figure out how to append the cell coloring formula onto the existing formula in each task bar cell.
Any suggestions (or hopefully .. solutions) will be greatly appreciated.
Mike