Game of Thrones Ice Dragon were the flames hot or cold?

Did undead Viserion breathe hot fire that melted the wall? Or were the flames super cold which shattered the walls?
I've been debating this issue with my fellow Game of Thrones enthusiast friends for months now and most of them cling to the 'hot flame theory'. Sources online seem to agree with my friends that the magical flames of undead Viserion was indeed hot fire. But damn , that seems like a plot hole! If the trend for undead wights is to burst into uncontrollable flames when touched by fire then how the hell could Viserion breathe hot fire without bursting into flames? I find that the argument of 'magic' seems like more of a plot hole than a reasonable explanation.

I say the flames have to be colder than ice!