Did Colombo drink alcohol? I think he drank Bourbon in Prescription for Murder, or did he accept drinks and then not take a sip. Did he drink wine at the bribery dinner with Brimmer? or with Paul Gerard?

Are there any examples of Colombo drinking alcohol whether on duty or not?

Yes there is. He drinks wine (and some other type of drink in a bar) in Any Old Port. There's one episode, I don't remember which, that he apologizes to someone's wife for being drunk and making a fool of himself at a police function(?) when she says she recognizes him. In Prescription Murder, he has a drink while talking with the murder in his office. I haven't seen that office in awhile, so I'm not sure if he actually takes a drink. He also has wine with the last dinner with Paul Gerard in Murder Under Glass.

He gets downright sloshed in "The Conspirators".

I think the one where he alludes to getting drunk at a police function is "A Friend in Deed".

Despite the many episode where he does take a drink, the ones that stick in my mind are those where he has a drink in his hand, goes to take a sip, and then stops in mid arm motion to make a point, as with Robert Vaughn in "Troubled Waters".

And then there's when he hits up poor old Mike Lally for a can of beer in "Now you See Him" and never drinks it!

I remember in 'Murder by the Book' the killer ( played by Jack Cassidey ) askes Columbo if he wants a drink. Columbo says something like " maybe i'll have a drop of bourbon". Then the killer says "bourbon?" almost like he didn't approve of Columbo's.

Many episodes Columbo refuses and says he's on duty.

In that bar scene of LADY IN WAITING, when he tells Leslie Nielsen he isn't drinking liquor, I could swear he also says "I don't drink."
Maybe I always hear it incorrectly, but if not, that's very inconsistent.

Columbo did rarely drink, but only when it was to his advantage to do so. Of course he sampled a lot of wine in that wine episode.

He may have carried a flask of Bourbon in his coat, that would explain his attachment to it...

He drank half of glass of Burban in Murder by the book in a scene in Jack Cassidy home