I consider it to be a pure green in the sense that it was a single green phosphor used in the display. It is true that in terms of light nanometers there are different shades of green and Apple's wasn't what RGB green is, but it was a fully green pixel.

I do own this exact monitor, I should recap it one of these days. The plastic is very yellow now sadly.

Quite a lot of phosphor formulations existed: https://en.wikipedia.org/wiki/Phosphor#Standard_phosphor_typ...