"Concurrent activites" is a military term, defined as an activity that continues throughout a project as other phases are completed. There are many different aspects to getting the Expeditor "serviceable", and the colour coding indicates the progress of each part of the plane.
The red colour shows when a section hasn't been started. The yellow colour shows when a section has been started, or that not all of the parts for the section have arrived, or the final inspection pending full assembly of the section has yet to happen. The green colour shows when a section has all the parts, the final inspection pending full assembly has happened, and the section in question is waiting to be installed on the aircraft.
Image taken from EO 05-45B-4 page 10. From the files of TTA.
