The user and format of this receipt are the same as ones with orange-yellow imprints, leading to the speculation that the imprint did not start out as gray. On the other hand, other users did color-code their receipts, including the revenue imprint, and this could be a case of such. Or the ink supply could have been depleted and a different color subsituted. Whatever its origin, the color has its own catalog listing.
Scan courtesy of Hermann Ivester.