These days, kids identify them by the aspect ratio.
When I was a kid I used to think black and white meant the TV show or whatever used to be in color but since it got old it turned black and white. My thought process was they changed color just like old people’s hair turns grey… This was 35 years ago before internet.
No, that was just for effect. Notice that all the scenes playing in Kansas are B&W (even the ones at the end), and all of Oz was in Color. It gave the place an extra kind of quality above the B&W pictures they were used to. I have heard that people in the cinemas gasped in surprise when the switch happened.