Which bar colours? The one that identifies the physical and mental strength of Bob?
If that is what you are asking about then this:
The bars increase with experience, making Bob stronger as he gets more experience. As he gets toward the end of his career these bars start to go down. Making him less strong physically and mentally. They will never go up again. He will still drive for you, but if you keep him on he will lose strength more and more.
My advice (if you already have the air of experience trophy) - retire Bob and get a new one.