When the Bureau of Engraving and Printing began producing the battleship series of tax stamps, the half cent documentary was orange. This is a great color for highlighting cancels and hence great for the cancel collector. However, there was trouble. Every stamp in the documentary series up to the 80 cent stamp had the exact same design as the half center, as did all 12 values of the proprietary series. The Bureau spread the available colors across all these stamps. Even then they ended up with piles of stamps that all looked the same.
This was particularly the case with the 3/8 cent proprietary stamp. Compare the two side-by-side stamps below. At first and second glance they look the same. The inevitable confusion wasn't just accepted. The Bureau went so far as to pick another color for the half cent stamp, and so they went with gray.