Fans who have been waiting for an epic showdown between ice dragons and fire dragons thought they might have gotten it when last week's signature penultimate battle episode featured The Night King transforming a fallen dragon into a wight. It's true that he resurrected one of Daenerys' fire dragons but, if I'm being technical, there's one huge hint that points to why Viserion can't be an ice dragon on Game of Thrones. In the books, ice dragons are described as being much, much bigger than fire dragons. And while The Night King can breathe frozen zombie life into the dead, he definitely can't resize his prey. (Although White Walkers do appear to "grow" normally, since he turned Craster's sons as babies.)
In fact, everything fans know from the books suggests that ice dragons are born, not made, though they almost certainly have magical origins. They are said to roam the Shivering Sea and the White Waste, breathing icy air that can freeze a person on contact, much like their fire-breathing cousins can roast someone on contact. Although the physical description of ice dragons having crystal blue eyes matches the creature viewers saw in Episode 6, "Beyond the Wall," ice dragons and wight dragons are definitively two different things in the books. Most likely, the television adaptation is just rolling them both into one idea for simplicity's sake.
Most of the information about ice dragons comes from a children's book George R.R. Martin wrote 16 years before the first A Song of Ice and Fire installment came out. It was called The Ice Dragon, and although it has plenty of similarities to ASOIAF, they do not strictly take place in the same universe. But we can still pull a lot of information about ice dragon lore from the book.
In it, an ice dragon befriends a princess and helps her defeat seven fire dragons. Afterward, the ice dragon melts into a small, cold pond, prompting some readers to believe that they can lie dormant in cold bodies of water, waiting to be resurrected. (One fan theory hypothesizes that the pond outside of Winterfell is an ice dragon lying in wait for Jon Snow to awaken it when the time comes.)
The most important distinction this draws is that ice dragons obey their human rulers, just like fire dragons. Meanwhile, The Night King obviously controls the wight dragon. Another fan theory hypothesizes that Bran might be able to warg into the wight dragon, either wresting control of it away from The Night King or at least predicting the damage it intends to inflict. But even if Viserion isn't an ice dragon, he can still do a lot of harm.