In Avatar: The Last Airbender, Zuko becomes the Blue Spirit, even though blue is not a typical Fire Nation color, because it's meant to be misleading.
In Avatar: The Last Airbender, Zuko takes on an alternate identity known as the Blue Spirit, but, considering that Zuko is a firebender (and red is more typically associated with firebenders than blue), why didn’t he become the Red Spirit? The Blue Spirit episode was meant to be a possible finale in case the show didn’t get renewed, but the Blue Spirit also marks an important turning point for Zuko’s character.
Avatar season 1, episode 13, “The Blue Spirit,” shows a mysterious figure in a blue and white mask rescuing Aang from Admiral Zhao. While Katara and Sokka are sick and unable to travel, Aang goes to look for a cure and is subsequently captured by Zhao. The Blue Spirit frees Aang, but while they attempt to flee Zhao, the Blue Spirit is wounded. Aang removes the distinctive mask and discovers that the Blue Spirit is none other than his enemy, Zuko. The reveal was a complete shock for Aang as well as viewers since Zuko was possibly the last person they expected to see beneath the blue mask. According to showrunners Michael Dante DiMartino and Bryan Konietzko, the color of the mask was integral to throwing everyone off.
In the book Avatar: The Last Airbender – The Art of the Animated Series, Konietzko reveals that the color of the mask was, in fact, originally red. Konietzko had come up with several concepts, researching various masks from Asian cultures along the way, and, unfortunately, ended up realizing that the red mask made Zuko look “a bit like an ancient Korean version of Spider-Man.” One of the directors, Anthony Lioi, also pointed out that the color red might give away Zuko’s identity since it’s so closely associated with the Fire Nation. For these two reasons, the color of the mask was changed to blue and it ended up working spectacularly.
However, there is still a Red Spirit that exists in the universe of Avatar. In The Last Airbender prequel novel, Zuko’s Story, Zuko meets Hong Shen, who is a sort of Robin Hood-like folk hero that steals from the Fire Nation military to help the poor. Zuko helps Shen and, in return, Shen gives Zuko his Blue Spirit mask and the dual swords that Zuko wields as the Blue Spirit. Inspired by his noble friend, Zuko takes up his Blue Spirit alter-ego, though his motivations for rescuing Aang may have been selfish.
Zuko, a Fire Nation prince, might have known that wearing a red mask would signify his nationality. Since he wears the mask explicitly for covert operations, Zuko might have chosen the color blue to distract Zhao, whom he tries to evade, and Aang, whom Zuko rescues with the intent of recapturing him. In the end, Zuko’s rescue of Aang leads to a heartbreaking moment when Aang wonders if he and Zuko might have been friends in another life and Aang clearly feels betrayed by Zuko – the last person Aang, as well as fans of Avatar: The Last Airbender, imagined the Blue Spirit to be.
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