When Game of Thrones viewers saw Viserion's eyes go blue, a few questions popped into everyone's mind. What does it mean for Westeros that the Night King now has a dragon? Could this undead Dragon topple The Wall? What will come out when Viserion opens his mouth? The answer to those questions, respectively are "nothing good," "Yes," and "blue stuff." Viserion's blue fire breath seems to be a devastating force of destruction but what does that mean? Is the blue fire he's spewing genuine fire or is it some kind of icy flame?
Game of Thrones has never been limited by what is and isn't possible in the real world. Dragons, zombies, and shadow assassin babies all populate the world of Westeros, meaning that Viserion could breath whatever George R. R. Martin wanted Viserion to breath. However, it certainly didn't look like Viserion was breathing any kind of "ice beam" or anything type of cold breath that would imply that Viserion is an ice dragon. Viserion is still a fire-breathing Dragon, although a dead one. However, that doesn't mean his blue fire shouldn't be feared. If anything, his blue breath proves that Viserion may now be more powerful than Drogon or Rhaegal.
While the real world doesn't have dragons (undead or otherwise), it does have blue fire. If fire works in Game of Thrones as it does in the real world, then the fire that Viserion is spitting is actually hotter than regular fire. As a flame grows hotter, its color changes from red, to orange, to yellow, to the bluish-white shade that is seen taking down The Wall. That means if Viserion were to go head-to-head with Drogon or Rhaegal, that Viserion would likely come out on top. It is said that fire cannot kill a dragon, but dragons have never encountered fire like this.
However, if we're applying science to Game of Thrones, it is worth it to ask how Viserion could breath fire hotter than a living dragon. Can dead dragons do all the cool stuff that living dragons do, like breath fire and fly, even if their bodies don't work or if their wings have a bunch of holes in them? Is the fire just a whole bunch of White Walker magic that the Night King happens to be using while flying on a dragon? The image of blue and white flames taking down the wall is cool, but it may not hold up scientifically.
Then again, assassin shadow babies also don't hold up scientifically. If the fire in Westeros works the same way as it does in the real world, then Viserion's blue flames are to be feared. There's a chance that the flames could also be a choice by The Night King to make sure the flames fit his color palette. No matter what color Viserion's flames are, from red to blue to mauve, there's a good reason to be scared of them. The icy white walkers have harnessed fire to take down the wall, and The Great War has truly begun.