I've heard the rgb color tv explanation before.
Of course in WNMHGB and maybe Cage, you had greenish/gold and a tan/buff, and blue. Was there red then? Can't remember.
You're remembering correctly, they didn't have the colors spread out in color space yet. It makes me suspect that RGB color TV's hadn't been taken into consideration to pin down the color choices, by that point in the development.
Or, maybe they had taken it into consideration, and the color choices were provisional, pending the results of the pilots, to see how well they photographed.
I'm just speculating here.