I heard that's because the network didn't know if the show was going to be a hit or not, so Sherwood Schwartz didn't want to spend the extra for color until the show was renewed. (Ted Turner fixed that in the 1990s by colorizing the first season, like he did to so many B&W movies.) After the first season, they also rewrote the theme song to add "The Professor and Mary Ann."
RIP Professor.
The American Flag in Honolulu Harbor is at half mast during the opening of the Pilot. Who knows why?
The pilot movie was never released, except on the DVD collection of the entire series which I bought last year.
It did not have the Professor, Mary Ann or Ginger. Instead there was a secretary and another married couple, younger than the Howells, perhaps mid thirties. After watching it, I can see why it was never released. It wasn’t all that funny and there was no clash between any of the characters. The parts of the Skipper and Gilligan were not as slap-stick as they eventually became. Had it remained as the pilot, it would never made it thru the first season...................
Bob Denver (Gilligan) threatened to walk off the show if they didn't give Johnson and Wells billing in the opening credits.