In this answer (S7E6 spoiler) the explanation is given that Viserion is the subject of that question, based on the colour of the dragon:
"It may be as you say, blood of my blood," Dany replied gravely, "but he shall have a new name for this new life. I would name them all for those the gods have taken. The green one shall be Rhaegal, for my valiant brother who died on the green banks of the Trident. The cream-and-gold I call Viserion. Viserys was cruel and weak and frightened, yet he was my brother still. His dragon will do what he could not."
"And the black beast?" asked Ser Jorah Mormont.
"The black," she said, "is Drogon."
A Clash of Kings, Daenerys I
However, that is based on their description in the books. Drogon is more explicitly named in the show, so there is no doubt about him, but does the show give any conclusive evidence on which dragon is which in universe?
And let's disregard subtitles (Season 7 Episode 6 spoiler):