The book series is one big loop, especially when you consider the last word mentioned in each book.
What was the last word in "Uglies"? Pretty.
What was the last word in "Pretties"? Special.
And what's the last word in "Specials"? Ugly. That last one is where it all comes full circle.
Ever notice that every time Tally changes sides or identities, it's because of Shay? It's like a pattern. Tally started out your every day believer in the operation. Shay wanted to be a member of the Smoke, and eventually (between blackmail and gradual defecting) so does Tally. Shay is forcefully turned Pretty by Special Circumstances, so Tally allows herself to be Pretty so her allies have a willing test subject. An embittered Pretty Shay upgrades herself to become a (Special) Cutter, she forces Tally to become a Cutter (possibly to spite her and keep her under her thumb). Ultimately, what breaks the cycle is that once she's cured of being a Special, Shay decides against curing Tally.
Fridge-Awesome: Why did Special Circumstances employ Tally to find the Smoke herself, when they themselves are supposed to be the closest thing to Secret Intelligence Operatives? They should be able to track them down using Shay's clues. But they don't! Had it not been for manipulating poor Tally into being their spy, they would've never cracked the code in a million years. Shay's cryptic instructions on finding the Smoke are just that good!
Pretties often receive fancy new eye colors after their operation. The most prominently mentioned examples of this in the books are Shay, Tally, and Zane, who are noted to have been given copper, silver, and gold eyes respectively. These specific colors bring Olympic medals to mind in a way that feels symbolic of each character's overall self-esteem. Shay is given third-place copper because many of her struggles come from losing out to her traitorous friend(?) and romantic rival Tally. Tally is given second-place silver because many of her struggles come from feeling like she's never quite good enough in her own skin. Only Zane, who tends to be more self-assured and confident by comparison, gets the first-place gold eyes.
