In Shakespeare's Macbeth, what is "nose-painting"?

The short answer to this question is that we don't actually know for certain what Shakespeare meant by this term, a colloquialism whose meaning has not survived. However, we can certainly speculate as to what it indicates, based on context. The Porter says that drink "especially provoke[s] . . . nose-painting, sleep and urine." Some scholars have suggested that "nose-painting" simply refers to the reddening of the nose which occurs alongside a generally flushed face when one has drunk too much, but this does not explain why the Porter then goes on to discuss "lechery."
He comments to Macbeth that drink "provokes the desire, but it takes away the performance"—that is, alcohol makes people want to copulate but also makes it difficult for them to do so. The positioning of this discussion strongly suggests that "lechery" has been mentioned in his original list of the things drink provokes; thus, we could assume that "nose-painting" signifies lechery and is sexual slang. There are a few other places in Shakespeare's works where "nose" seems to have a sexual connotation (Othello says to Cassio, whom he believes to be sleeping with his wife, "I see that nose of yours, but not the dog I shall throw it to," for example). So we may infer that, if "nose" means "penis," "nose-painting" is a slang term for copulation. (Compare other slang terms involving the insertion of a pen into an inkwell, for example. You can probably think of several modern slang terms which are similar.)

In Act 2, Scene 3 of Macbeth, on the second page, the Porter says that drinking provokes "nose-painting, sleep and urine." While we know what the last two things are, it is a little more unclear what the term "nose-painting" refers to. While many people assume that it means the turning red of your nose when you drink, other sources point to the real meaning being a little more bawdy than that.
Shakespeare uses many euphemisms to refer to sexual acts, and this might be no exception. If we look at the context surrounding the quote, we get a clue as to what it might signify: "Lechery, sir, it provokes and unprovokes; it provokes the desire, but it takes away the performance... makes him stand to and not stand to." If we take this to mean that he is using these phrases as a metaphor for sexual behavior, such as the desire to perform and not to perform and the ability to "stand to" (achieve an erection), then we might assume that painting one's nose could refer to some sort of sexual act. 
Furthermore, the term "lechery" is used, and this is a strong indicator of a sexual reference. Lechery is defined as excessive sexual desire, or lustfulness. This seems to suggest that although the drunk person in question has the desire to engage in sexual activity, he will be unable to "get it up," so to speak, and will thus be unable to perform sexually.