there's a lady with purple outfit also in the airport scene, again, this refers to the jitterbug dance at the beginning of the film
You're referring to the purple background we see in the opening dance seen. At times it looks like purple splotches spilled onto the dancers (red arrows).

Upon careful inspection, we get the impression that the purple background is a curtain that the dancers (and their shadows) can pass behind and disappear from view. It could be a metaphor for conscious vs subconscious, or even life, death, and rebirth.
Notice, that what looks like a room full of dancers is really only three couples in duplicate/triplicate. We hear a swing soundtrack that reminds us of the 1940's as does their style of dance, the jitterbug. I also have the impression that I'm hearing cars honking their horns. Eventually, Betty/Diane comes into focus. The spotlight is so intense, she is almost pure white.

In my view, what we don't see is probably more important. We don't see Betty dance and we don't see her partner. Instead, for brief moments, we see a blurred, semi-transparent image of her standing between an elderly couple.

Some believe that they are judges, others believe that they are her parents. Both ideas can be true. For example, we can imagine that Betty has judgemental parents. As for my overall impression, I get Shakespeare:
"Tomorrow, and tomorrow, and tomorrow,
Creeps in this petty pace from day to day,
To the last syllable of recorded time;
And all our yesterdays have lighted fools
The way to dusty death. Out, out, brief candle!
Life's but a walking shadow, a poor player,
That struts and frets his hour upon the stage,
And then is heard no more. It is a tale
Told by an idiot, full of sound and fury,
Signifying nothing."
Betty is the candle, and the walking shadows loom larger than life behind the dancers. The purple background is the curtain from the performers makes their entrances and exits.