I don't know if it's written in stone anywhere, but a quick review of the scene looks like it's the inverse of what you're saying.

To me, it looks as follows:

Shot 1: Judy was in Sepia. Part of her face was showing in that shot (albeit with a creative use of bad lighting), her double in color stepped out, but as a view from behind, obscuring her face.

Shot 2: A head-shot of Judy as Dorothy from the reverse angle was spliced in to show that it was her. But... oh look, her hands moved. Clearly, this was a distinct shot. I would expect a multiple-camera setup for a shot like this to preserve continuity, which begs the question of why a whole separate take with continuity errors was leaned on in order to accomplish the effect if that was actually Judy who stepped out in color.

Shot 3: A continuation of shot 1 (the hand placement and camera position are exactly where you'd project them to be with approx 1/2 second of missing film in between) but the face of the "color" version of Dorothy is again obscured. There is a creative use of foreground and a long-drawn out take (misdirection) until Judy's double is sufficiently far from the lens so as to be obscured via good old-fashioned limitations of focal length.

Shot 4: As before, definitely not from a continuous take, begging the same question as before.