So Spectra 6 makes black, white, red, yellow, blue, and green from white, red, yellow, and blue primaries. So green is presumably blue plus yellow? If so, why

So Spectra 6 makes black, white, red, yellow, blue, and green from white, red, yellow, and blue primaries. So green is presumably blue plus yellow? If so, why not get purple from blue plus red? Presumably some additive versus subtractive thing but I don’t get it.