This is spoken about explicitly in the script for the final episode of the season, released because it is up for an emmy.
A massive chunk of the Wall breaks free ahead of Tormund. The ice dragon glides away, and we get our first clear look at him, and at the Night King on his back. He's done the same thing to Viserion that he did to Craster's sons. Only those were babies, and this is a dragon.
Game of Thrones, Season 7 Episode 7, "The Dragon and the Wolf"
I've covered the distinctions between the three of them on my answer here but as a quick run down:
- Drogon: Black
- Rhaegal: Green
- Viserion: Gold
Looking at the dragon that was killed it is then clearly Viserion as it is neither black nor green so that leaves that one. It does look yellowish too but because of the blueish tint to the whole episode it's hard to tell.
On an out of universe reasoning HBO has released this 3D card titled "Game of Thrones The Night King and Viserion 3D Card" so It clears it up too.