What't the meaning

Paint the town red

Meaning: To party and celebrate with enthusiasm in bars and restaurants.
Example: At the end of final exams college students have traditionally gone out to paint the town red. College students traditionally also do this at the throughout the semester.

Origin: This phrase originated with the Roman Empire. Roman soldiers used to wash the walls of a newly-conquered town or city with the blood of the vanquished. This was usually accomplished with a great degree of gusto, hence the term being applied to a great night on the town.

Thanks to Gregory Pius

Alternatively,

"Paint the town red" is a reference to the money spent in the process. The party goer ends up in the red paying for the celebration. See the related phrase "in the red".

Alternatively,

A joke that the resulting blood-shot eyes resulting from partying and drinking might actually color one's field of vision red.

Thanks to Ron Akers

Interestingly the movie "High Plains Drifter" starring Clint Eastwood actually had a scene in which the entire town was painted red, literally. In this Western, Clint played a tough character who sought revenge on the town folk that had wronged him.

Clint basically manhandled and had his way with the entire town. In a dramatic scene he had them paint all the buildings red - and change the name of the town to Hell.