[![Glasses of Romulan Ale being poured][1]][1] Romulan Ale is blue.

One of the first instances of seeing it is from *The Wrath of Kahn* 

> Admiral James T. Kirk received a bottle of Romulan ale, dated 2283, as a birthday gift from Doctor Leonard McCoy in 2285. (Star Trek II: The Wrath of Khan) [MEMORY ALPHA][2]


  [1]: https://i.sstatic.net/8rXWm.jpg
  [2]: http://memory-alpha.wikia.com/wiki/Romulan_ale