Ok, as someone who once did a graphics design qualification, I have watched both a few times and asked myself "If I was the graphics person for both shows, why would would I design it that way?"
In the first case of The Brady Bunch the same shade of blue is used. It's a pretty calming shade. That combined with the squares produces a happy album or happy photographs effect. That would seem pretty appropriate for such a sitcom.
The Gilligan's Island Graphics are to me more interesting. Different shades of blue are used for different things: there's the threatening darker blue/black of the water. Then there is the calmer blue of the round character decals. Interestingly the boat itself, shown briefly, is a very light shade of blue that suggests that it's a place of safety on the boat. But then the boat on the shore is white; I don't know if that was a mistake or something deliberate. Note that when the decals start some of the people are wearing blue clothes so that is a bit more organic; they are part of nature the sea etc, rather than The Brady bunch who were optically distinct.
So in both I don't think that the greenscreen was used without other elements of shape and colour being made to fit ideas and themes.