Symbolism, Imagery, Allegory
What's in a dress? Each sixteen-year-old girl gets to choose a fancy dress from an allotted selection to wear to their Match Banquets—it's the one time they get to wear something besides plainclothes. Cassia picks a green dress, and is told that she is "the only girl wearing this dress at the Match Banquet this month. The most popular gown is one of the pink gowns, number twenty-two" (3.12), which is not surprising because she's "picked things outside of the majority in the past" (3.10). Okay, so our girl likes green and she's got her own sense of style.
What the green dress tells us is that even before the snafu with her Match, Cassia was someone who didn't just follow what everyone else did. And the color green is often used to symbolize life, growth, and energy—all things that Cassia possesses, especially by the end of the book. And you know what else is green? Leaves—a.k.a. the things you find on trees… so now might be a good time to hop on over to the analysis of maple trees elsewhere in this section.
Even though she has to give the dress back, she gets a tiny scrap of it to keep afterward and remind her of that night. Once Society reclaims everyone's artifacts, it is the only thing she possesses that is uniquely hers (well, aside from whoever else has worn that dress). So when she's forced to say goodbye to Ky, she gives him her most prized possession, the only tangible piece of her own identity she can offer—the dress scrap. She may have worn it the night she was matched with Xander, but she's giving herself to Ky.