"Plum": a standard (but scarce) Parker 51 color
There has been a truly astonishing amount of misinformation about the 51 color, "Plum". A deep, dark purple, Plum was a regular production color for Aerometric-series Parker 51s in both regular and Demi sizes. It is listed in Parker's official catalogs, parts lists, and repair manuals of the era, as well as in the original production records preserved in the Parker company archives.
For whatever reason, Plum did not prove popular, so 51s in that color are scarce today. The color did remain in production for a number of years, but was discontinued well before the 51 itself. Not all Parker 51 literature of the 1950s mentions Plum, but it would be absurd to argue that this is indicative of anything other than the color's relative unpopularity and, as a result, its increasingly marginal status in the 51 line.
Plum is easily confused with the much more common burgundy in normal indoor light. In sunlight or under direct lamplight, however, the difference is obvious: Plum is clearly and unapologetically purple, while burgundy is red-brown. Despite what some have claimed, the range of variation in these colors is minimal, and in no way brings them anywhere close enough for confusion. The only USA-produced color in the Aerometric series that shows substantial variation is grey, and that chiefly in tone and not hue.