As nearly as can be determined at about the same time they switched from the Royal Gorge herald to the flying Rio Grande. One driving force that most tend to forget is that the government took a monopoly on green pigment for military purposes during WWII. As an example, the Lucky Strike cigarette pack used to be green with the red bullseye but in 1942 "The Lucky Strike Green has gone to War!" was the battle cry and the package remains white to this day. I suspect "Rio Grande Green" went to war as well and didn't return with the troops.