Because it's not really true what the poster says. You need Cyan, Magenta and Yellow as primary colors to really get the red, blue, green, yellow etc of the rainbow. For example, red is not "primary" but a combination of Magenta+Yellow.
Violet is notoriously difficult to produce, but in this case I think it may be due to fading. Each of the primary ink colors fade at different rates so complex colours will go through various color changes as the component colors slowing all give way to white.
Nice, thanks!

What's the ASBESTOS under-printed behind the masthead at the bottom about? Is that part of the original or added later?
Johns was the precursor company to the Johns-Manville asbestos conglomerate. The modern Johns-Manville entity was the result of the merger of two early construction products manufacturers. At the young age of 21, Henry Ward Johns had already patented asbestos roofing and insulation products. He founded the H. W. Johns Manufacturing Company in New York City.

In 1901, H. W. Johns Manufacturing and Manville Covering Co. merged to form H. W. Johns-Manville Company. In 1926, the company was renamed Johns Manville Corporation.

We all know what happened to Johns-Manville in the end.
You kids remember learning about Roy G. Bov in school?

Red, Orange, Yellow, Green, Blue, Olive, and Violet.