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Wouldn't the aliens simply have been able to replicate 3d specs whilst earthbound?
It really requires two eyes to see 3D. The aliens had one eye each.

There's no special connection between red and green and 3D. Any two colors that contrasted enough could be used in anaglyph 3D, which is the lousiest 3D process generally used. NONE of the 1950s 3D features were released in anaglyph; all were in the Polaroid process. Years (and years) later, a few of them were put out in anaglyph, including a double bill of IT CAME FROM OUTER SPACE and CREATURE FROM THE BLACK LAGOON.