Please take a read of these guidelines for some helpful advice on your first posts here on the forum - Tableau Forum Guidelines
To help you, we're going to need you to upload/attach a Tableau Packaged Workbook including sample data.
I see you've edited your original question and added some detail including a screenshot. There are loads of threads on the forum discussing the use of traffic lights / KPI's, please take a look at the search results -
After you've taken a look at those if you're still stuck, please upload a Tableau Packaged Workbook including your sample data and let us know where you've gotten stuck.