Thinking about the epic scale that the original Avatar: The Last Airbender achieved via animation, two images come to mind, both from the series finale. The first is the way Fire Lord Ozai’s lightning changes the color of the sky from its moody red hue to a dark green as he battles Aang with all of his might.
The second is the clash or Azula’s blue fire versus Zuko’s red in their climactic showdown. At the peak of firebending ability, thanks to Sozin’s Comet, their vibrant blasts swirl around each other with a sort of poetry and musicality that makes the flames seem alive, mirroring the lifelong clash of the siblings to create a mood both desperate and melancholic. The emotions in both of these moments, from the fear and awe at Ozai’s power to the chill-inducing tragedy of two young warriors battling for the soul of their homeland, isn’t just derived from the story and characters (though there’s no shortage of brilliance there); it’s also the dramatic shifts in color and the fluid, melodic animation. It’s a mood live-action cannot replicate.
Source: IGN.com Does Avatar: The Last Airbender Really Need the Live-Action Treatment?