I still do not know what the yellow wire does but I know it is not the motor starter, that is the red wire.
I could not figure out what the problem was today with my high pressure soap and wax and I knew it had to be a Coleman setup misunderstanding. So I called Greg and he answered my question over the phone! The problem I was having was I was getting pressurized water from the water line where I expected gravity feed. Greg explained to me the way the Coleman unit works, it uses cold water at city pressure for rinse and hot water or gravity feed for high pressure soap and wax. My problem was exactly what Greg told me over the phone and was a miswiring. Thanks Greg.