Murphy was right --
Polarized plastic has a two major problems. First, the plastic sheet on the windshield/canopy usually has to be curved which makes the polarization vary from angle to angle and creates light spots where external cues can be seen. Also, many windscreens have stresses in the glass which can be seen with polzatized goggles. Blue-amber works just fine. The instructor/safety pilot can see with only a minor effect.
Well, maybe not just fine. I tried it once in an airplane with some LED frequency/DME displays (which were amber), With the blue goggles, you couldn't read them at all
I used blue-amber to block the external cues in early HUD flight testing. By putting the amber sheet behind the HUD, you could read the HUD but not see any external cues.
While any complementary color pair will work, one must be careful. In one HUD program, we used red plastic on the windshield and a green visor. I was flying with the green visor and the safety pilot (flying with a minor color vision waiver) commented that the red sheet on the windshield was no problem -- in fact he couldn't tell it was there --- UNTIL --- the sun went behind a cloud and his view went from transparent to opaque! It is very disconcerting to hear your safety pilot say "I can't see anything!"
I agree with the basic thread -- we seem to have lost basic instrument flying skills.
Goldfish