Symbolism, Imagery, Allegory
Ah, the color white. So many things come to mind: virginity, purity, cleanliness, freshness, a new start, and, um, ghosts? Allow us to explain:
Tita stood as if in a trance, staring at the whiteness of the sheet; only for a few seconds, but long enough to cause a sort of blindness […] when she looked at Rosaura […] she saw only a snowy ghost. (2,114)
This is the moment in which Tita is forced to look the truth in the face and it ain't pretty. It's one thing to make a wedding cake for your sister and true love's wedding, it's another thing to see the sheet specifically made for the let's-get-it-on moment.
It makes total sense that Tita would be traumatized by this image. That sheet means the end of any hopes of being Pedro's wife, and the end of any normal relationship with Rosaura. It also means complete powerlessness—Mama Elena chose the fate of the two sisters, as well as Pedro.