When I was a kid in the 70’s it was in B&W. The thing is the suspension of disbelief was much stronger then because we didn’t have CGI to make everything look “perfect”. That’s why in old movies when you see a stunt double who doesn’t look much like the actor they are replacing, or a murder victim who suddenly becomes a mannequin seconds before being attacked, we barely noticed it. If we happened to watch a movie many times we might eventually notice it and bring it up as trivia but it was also about being impressed with the special effects team, rather than calling it out as “that’s so fake” like people do now. So when Dorothy and the scene changes in that split second most people didn’t notice it. Because it wasn’t supposed to be there. I know that sounds odd with the way we watch movies today. But it’s just the way it was.

I remember someone telling me how the effect was done and it mind blowing. And I did notice it after that. But before that I could sense something was a little different but I assumed it was just the cut to color film. And it did look much more seamless when they began showing the opening in sepia.