
Genuine stamp on the left; philatelic forgery on the right.
The green color was certainly used for printing the $1 documentary stamps at the outset in 1917, but by the 1930's yellow green was the color being used. When it was time to create the silver tax stamps of 1934, the yellow green version was what was used, the left stamp in the image being the genuine RG13. In order to fill those empty spaces in collectors' albums, someone obligingly created some RG13's (?) by taking some of the green $1 documentary stamps and overprinting them, a philatelic forgery, as seen on the right. They even went to some extraordinary lengths to recreate the collector of internal revenue cancellation to make these stamps look authentic. Alas, they only created half a cancel and I have (not illustrated here) an example where the cancel stops before it gets to the edge of the stamps!
Inexplicably, the Scott U.S. Specialized still lists the underlying color of RG13 as green. That should be yellow green.
Ron Lesher