Stamps of the earlier eras were color coded. The United States first class postage rate stamps of the Nineteenth century were red-brown (the overseas stamps were blue). So were the first class postage stamps of Great Britain, Canada, and Austria. The reason had nothing to do with design or aesthetics. Red dyes were cheaper. Here’s why.
Inks are a combination of a coloring agent and a solvent to move that coloring agent onto the medium that it is printing. Coloring agents can be either organic or inorganic. Inorganic pigments are expensive and often change color over time. The earliest stamp printings were in the hundreds of millions, and the postal authorities expected the plates that these stamps were printed from to be used over again over many years while the stamps were in use. They needed a cheap, commercially available ink that could be reordered in quantity and which would be pretty much the same exact shade over the many years that the stamps were being printed.
Red dyes were among the first pigments that humans created. The reason was simple. Oxygen and iron, the two components of red dies, are the first and fourth most common elements in the earth’s crust (the second and third elements—silicon and aluminum—can’t be formulated to produce a useful pigment). Red Dyes were among the earliest body and face dies produced by primitive man, and in some places even the mud itself dries to a pretty good red-brown dye. It was cost alone that made early postage stamps the color that they were. Red-brown was the cheapest. Chemically it is called anhydrous iron oxide.
For over a hundred years, stamp design critics (and stamp collectors of every generation have always criticized stamp design) have disliked the color and monotony of earlier US stamps. In the early twentieth century, collectors wanted more bi-color issues which, due to the primitive printing methods of the time, meant two runs through the press which led to invert errors and registration problems (registration problems are when the the center of the stamp is not centered within the vignette). Critics felt that more colorful stamps would attract more collectors to the hobby. The Bureau of Engraving and Printing didn’t see it as their job to entice collectors to the hobby, and it remained for technological changes in printing to make more colorful designs possible. It doesn’t seem that it was color that was keeping collectors from the hobby, as even with the most multicolor of designs collecting rates in the general population haven’t increased.