That article makes little sense. ROYGBIV is not accurate anyway on how we see. And Indigo is pretty much purple, and I have never seen indigo in the rainbow.
In the world of photography, and darkrooms we controlled the color of a photograph with cyan, magenta and yellow filters, and cyan was pretty much never used at all, just the magenta and yellow. Magenta and cyan are not even in the ROYGBIV named colors of the rainbow.
And in darkrooms, yellow and blue are opposite colors. In paint adding blue to yellow make green. That is additive color, where we see more in the subtractive color spectrum.
This is the color chart we see. Add a bit of magenta and a bit of blue, and presto purple, or in photo lab terms, magenta-blue.
With just magenta and yellow filters on the enlarger, we could color correct any negative to make a proper color image, assuming the negative was processed in good quality controlled chemistry.