Glinda is floating in a bubble.
Red smoke, like blood, is threatening.
Yin and Yang, light and dark, generous and greedy, angelic and wicked.

Ever notice that the yellow brick road starts in a golden spiral?

It is just a concept dreamed up by the special effects crew.
The availability of color film had a lot to do with it. They focused heavily on the transition scene, devising ways color could add depth, texture, and a sense of magic to the big screen.

Glinda encouraged theft of property, sent a confused, out-of-place young girl on a dangerous journey through a land she didn’t know, and manipulated that girl into murdering her rival. “Good” witch my ass.

Red is a color signifying danger and conflict, emotional responses that are associated with the bad witch. The good witch floats in on a symbol of purity and light and clean dishes.