I believe green was simply the first kind of phosphor available way back in the 1800s when cathode ray tubes (CRTs) were first invented, and they just kept using it all along because it was the cheapest. It's the same thing with LEDs: red LEDs were the first kind to be invented, and now even that other colors are available, red LEDs are still the cheapest and most widely used (although green is a close second).
I think the whole thing about green phosphor being "easy on the eyes" is either purely coincidental or just plain junk science made up to justify the continued use of the cheapest phosphor color available. In fact the extreme contrast between the bright green letters and the black background has a rather dazzling effect; with a high enough refresh rate, black text on a paper white background certainly is less agitating to stare at all day.
The most annoying phosphor color I've seen on monochrome CRTs is light blue (cyan), which I've seen on Amdek mono composite monitors (also available in green and amber phosphor) and some old ATMs (now probably long out of service).
My favorite was an IBM paper white mono VGA monitor, model 8503 I believe. Unlike the classic Macintosh monitor which has a bit of a blue/cyan tint, IBM's white phosphor has a slight off-white "eggshell" tint, which was very pleasant to my eyes, especially with VGA's relatively high resolution.
Outside of CRTs, the worst of all time has to be the reddish-orange gas plasma displays commonly used on "lunchbox"-type portable computers in the late '80s. Staring at one of those makes a Tandy VGM-200 (VGA color with 0.52 mm dot pitch) look beautiful in comparison!