OK465
I believe current HUD displays are monochrome because any color reflected back to the pilot's eyes by the combiner is also degraded externally when entering thru the combiner.
You could have a multi-colored SVS HUD, and then you could actually eliminate any windows on the flight deck, not much light coming thru anyway.
Right. There are some good reasons for choosing monochrome and green. The combiner can be optimized to reflect one color and only attenuate that color coming through the windscreen. Green is a good choice for night vision anyway. But when you step up to an SVS system with a monochrome projector, you end up with green symbols on top of green video. And that can look a bit jumbled.
The original logic behind selecting green was that for vector displays, the refresh rate varies based upon the number of symbols. Past a certain point, the image will flicker unless high persistence phosphors are used. Same thing holds true with old oscilloscopes with slow sweep speeds and slow refresh computer CRTs (all old tech). Green was the easiest color to work with.