We've had HUDs on airliners for well over twenty years that have got progressively more & more sophisticated but still with only one color - green. That's because the reflection HUD uses a single frequency holographic mirror.
Given the main theme in this thread,the HUD is possibly the one modern display that doesn't do Unusual Attiudes (UA) very well. You can have sky pointers, declutter at set Pitch & Roll angles and include ladders and lines but it's still not easy to decide which way is up - especially when you're in a total whiteout with no outside clues.
The other problem when you're only allowed a single color is that it's relatively easy to draw something that actually will "pop" on you such that the shape goes inside out as you blink.....