You are doing fine, almost there. If you delete the second line, than the first will work, only showing green cells where their content is IP2. CellStylerGrid gets confused with multiple rules for one column11 apparently.
But you only need one rule:
return ((rowObj.get('Overall_Industry_Priority')==='IP1')||(rowObj.get('Overall_Industry_Priority')==='IP2'))
As you probably figure: || is javascript for ‘or’.