Is changing the HUD colour realistic? I assume that changing the HUD brightness (different brightnesses of green) is realistic but on a real Superhornet HUD can you actually change to other colours like red etc...?

What about newer gen aircraft like the F-35 or F-22?

I don't remember properly, but I think I read somewhere (probably here) that the different colours available were simply to make the HUD more legible for the armchair pilot. In the actual aircraft, there's just green and you have contrast settings.

I'm not sure about 5th-gen aircraft, but I don't think they're an exception. After all, if a "HUD colour" option was really needed, they would definitely fit it to 4th-gen aircraft as well, but all HUD videos I've seen are in green, be it for the Hornet or the Raptor.

That makes sense, I thought the colours are unrealistic but I have to admit though, it sure helps in a lot of situations to change the colour to be able to see the HUD in the sim...

Yeah me too, and I still wonder why they didn't actually include the analog contrast setting instead of these colours.