Ray,

Red pigments are one of the earliest color pigments that human used. Red pigments occur naturally in the form of iron oxides, hence the name "oxide red" as a common paint color for that boxcar red sort of color. Of course, these colors have brown and orange overtones, so they are not the "pure" reds that one associates with the color "caboose red". One of the first uses that I am aware of for a "red" is the PRR's red background for keystones on passenger engines and signs. This color was called "toluidine red" and its use began in the late 1920. The paint was expensive and therefore unlikely to be used for the exterior of cars. Widespread use of truly red car paint seems to coincide with the introduction of synthetic paints and pigments following WWII.

Regards,

Bruce Smith

Auburn, AL