Why was the first part of the Wizard of Oz in black and white and the second part was in color?

It was done to heighten the fantasy atmosphere. the film started out in Black and white and when the Cyclone hit- then there was another torrent of technicolor- spring on the audience- odd and unreal colors were transmitted ( probably using follow spot strobes) on horses , for example. I had only seen the picture on TV in black and white and was unaware of this (rainbow) though not called, transition.