Between the black and white and color episodes of I Dream of Jeannie.

Tony mentioned in "Happy Anniversary" that she ''got a new bottle'' when he picked up the Blue Djinn bottle. But that design was a dirtied up season one vines bottle. And earlier in that episode he knew she now used a purple new design. But that episode was not the first one filmed for season 2...so there never was an explanation. Other than the production team wanted to take advantage of going to color that year.

Al lot changed when the show went to color, i.e. Tony's house design, Jeannie was a genie from birth and not turned into one by the Blue Djinn, Jeannie herself became dumbed down, where as in the early episodes she had good sense.

Just seeing the thread title, my first thought was the switch to color, too.

The switch to color is what created the new bottle