Formula to change RYGG balls when Project Stage changes


I am new to SS and I am trying to automate the Project Status Red, Yellow, Green & Gray balls to change to the correct color based on the Project Stage selected. 

Project Stage drop down will need to work as follows:

Red Ball = Not Started

Yellow Ball = Scheduled, Field Work, Drafting, Completed, Preliminary Sent

Gray Ball = Invoiced

Green Ball = Paid

Does anyone know the correct formula to make this happen. Thanking you in advance. 3



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