Why did Gilligan's Island and The Brady Bunch intros use blue screens?

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.

Coincidence? Consider that both were created around the same time (GI from '64 to '67, BB from '69 to '74) by Sherwood Schwartz. Schwartz only created one other TV show, It's About Time, which aired from '66 to '67. Notice anything about the intro?

At least for the Brady Bunch, it looks like the board from the game show Tic Tac Dough, which had blue backgrounds on the squares. I always thought it meant to match that.