Cost. The mold for the "Astronaut" head was made in 1972 and hadn't been used yet. Why retool another mold for a generic head when you've got one already to go?
I was thirteen in 1974, so I was old enough to know the suits were supposed to be white, but it didn't bother me that they were blue. Mine wore primitive custom outfits.
Again, you're probably right: They just made a "simple" figure rather than even bothering with a white suit, etc... But, to give some kids credit, I'm sure there were a few that thought "Why's this guy in blue?" I say that because PLANETand BENEATH were on TV before the figures even came out and I'd guess at least a few kids seen them and kept that white outfit in the back of their head... Plus, for the kids that were a little older (probably early teens by the time the figures came out), they might have had the TOPPS gum cards lying around, and those cards do show the guys in their white space-suits...

Comment