The color coding in understanding in project dash board vary form organization to organization. However the common understanding in most organization is
Green : The project is under control and we can deliver the project as per the schedule, with in cost agreed and quality
Yellow: The project is under control but have some problem with schedule, cost or quality. However there are high chances to bring the project under control by re-planning.
Red: The project is not under control. There is serious problem with schedule, cost or quality. Immediate attention needed from higher management to bring the project under control if possible.