The backseat HUD is pretty neat. I did a backseat ride about 8 years ago or so. I had a lot of HUD time and so was quite happy jumping in the back and looking through the HUD. All looked normal except for the funny green colour over the whole screen. I finally asked the guy in front about the colour and mentioned that it was just how it came through from the front. I didn't really understand until it clicked that I was sitting in the backseat and looking at the hangar in front of me. I moved my head to the side and of course right in front of me was the front seat. For the whole flight I really did feel like I was sat at the front. Great piece of kit.
ps. Just to be clear there is a camera at the front of the jet that projects its image onto the rear seat HUD. Not only do you see normal HUD symbology but also a video image of the forward view.