In her 20s, she probably would wear No. 37 Veilchen (Pure Violet) perfume by Frau Tonis. The brand was just resurrected by the owner's granddaughter in 2009; they write on their website:
"Now to our final question: You also offer scents that once enthralled the legendary Marlene Dietrich?
Yes, that is correct. Dietrich’s favourite perfume was a scent as intense and eccentric as her she herself: pure violet. A scent that is dominated by the sweet, intense almost stubborn top note. This perfume may not capture the spirit of the time today, however, one can easily imagine how Berlin’s grande-dame of the roaring 20s once used this scent to cause quite the stir."
So I had to visit them.

I've bought the smallest version of this scent--it really is violet in all its glory--if you've ever drunk Creme de Violette from Monin, then the smell is exactly the same. I myself can imagine Marlene wearing this perfume in the 1920s Berlin; it's a bit sweet, intoxicating and very extravagant, but not as sophisticated as Dietrich's later olfactory choices".

Пронзительная фиалка. Это самая горькая и самая зеленая фиалка из всех фиалок которые я могу припомнить. Пронзительная горечь зелени бьет наотмашь при первом вдохе. Не фиалка – вопль отчаяния. Она смягчится постепенно. И все равно останется горькой и зеленой. И в конце жизни это уже не фиалка, а корень ириса – горький и зеленый. Это потрясающий аромат. Это одна из лучших фиалок. Это фиалковая трагедия во флаконе.