Why is there 2 versions of this war bonnet? One has lighter blue and yellow and the other has a darker blue and brighter yellow color scheme. Is there a difference in the way they operated?

Wondering when the Santa Fe Frieghtbonnet Blue/Yellow color scheme starting disappearing from main line service. I've been told it started in 1972, but don't remember what time period they started to disappear. My best guess would be late 80s, early 90s. By that time the Warbonnet dash 8s were...