Colouring a VIZ bar chart based on the measure

Hi experts ,

I am trying to generate a viz bar chart .

     Say it has 1 measure (Marks) and 1 dimension (Students) .

Now if the marks is greater than 80 the bar color should be green otherwise red.

I am refering JsDoc Report - SAP UI development Toolkit for HTML5 - API Reference - sap.viz.ui5.Bar

In the plotarea I am using formatRules  JsDoc Report - SAP UI development Toolkit for HTML5 - API Reference - sap.viz.ui5.types.Bar

But no luck with that.Is that the right approach ?

I also tried with formatter . Code looks somewhat like :

colorPlatte : {

                         path :  "marks",

                         formatter : util,formatter()

                    }

but that didnt worked too .

Any idea what is wrong??