Why do Carmelite nuns change headwear? In this movie regarding the life of St. Therese of Lisieux, the girl who wants to become a saint, there is a scene wherein she changes the color of her clothing and headdress after participating some time at the monastery. What is the meaning of this color change that marks her becoming a nun?
The first official stage of the religious life is the noviciate.
Novices are not admitted to vows until they have successfully completed the prescribed period of training and proving, called the novitiate. This usually lasts one year.

This is the period that the member of a religious community undergoes prior to taking vows (poverty, chastity and obedience) in order to discern whether she or he is called to vowed religious life.

The novice's habit is often slightly different from those of professed members of the order. For instance, in communities of women that wear a dark veil over the head, novices often wear a white one (like the case of the Carmelite Order).