Justification of the choice of colours in monochrome displays
Quick bit of Wikipedia leads to: http://en.wikipedia.org/wiki/Phosphor#Standard_phosphor_types

"P1" is a zinc silicate which is found natually occuring in an ore. I suspect this was the first kind of phosphorescent material to be used in CRTs, and as it's naturally occuring it may be the cheapest. In most monochrome CRT applications (especially osciliscopes) there's no need to upgrade to white and people are familiar with green.