First early consumer monitors were just Black & White TV components, with a white or bluish tinge, kinda ghosty and harsh with all the brightness, or weak without being bright.

Then came 3rd party glare filters to put over the screens which had a smoke or polarizing layer which helped a lot (could ramp up brightness and not have the ghosting), many of these were green (some were mesh fabric). Not soon after monochrome displays were out with green screen coating.

Later there was a trend for amber display, I belive it was because it was more contrasty than green, which people working with spreadsheets and higher rez (smaller character) displays. Some bought them because it was hot with business culture. I know some people really preferred it though, but green was fine for me, seemed more soothing than amber.

So for classic websites I see a lot more green than amber (textfiles.com comes to mind), but it might be related to what the web designer was used to in the past.

Phosphor mixes changed over the years. Initially white phosphor was the thing everyone used because TV, you could do grey scale (aka black and white TV) with one phosphor.

This was expensive stuff (relatively as it needed reasonably linear response to beam energy) and in test equipmentment (like oscilloscopes) green phosphor came into use. Both because it was monochrome anyway, and because it had higher "persistence" which was bad for video but good for things where you wanted to see where the beam had been.

I recall when DEC introduced amber phosphor terminals in the mid 70's arguing that they ghosted less (lower persistence) and they were easier on the eyes (less eye strain). Full color terminals followed with the growth of color TVs.

I'm not sure why green phosphor was the way to go in the early days. But I do remember that companies that saw themselves as progressive switched to amber because it was believed to reduce eye strain.

I think the green phosphor gave more (apparent?) light per watt.

When I was a kid I often visited the family firm where a lot of users performed various functions on an S/38 mini-mainframe by means of dumb terminals connected with Twinax (I was seven or eight, I don’t remember the part number of the terminals).

Anyway, long story short: everybody had the traditional green-phosphor-on-black displays, except one person, who had an orange-phosphor-on-black display. When I asked up-front about this (as seven/eight year olds are wont to do) it was explained to me by this person that they were colourblind and that an orange phosphor display had been procured for them to make the experience more pleasant.

For me it's a sweet spot for contrast levels.

Black on light blue is also pretty good.

I use solarized light : https://ethanschoonover.com/solarized/

I'm totally spitballing, but reduced contrast can be a little less harsh than pure black on white, which might be a motivation. Other than that, perhaps it is meant to mimic the experience of reading a book by a warm light.

It's pretty easy reading, close to a book? (Closer than 'computer white' anyway, obviously paper varies.)

in nature (stinging insects and large cats) this is a signal of DANGER