Why is it appropriate that Lady Macbeth's response to Duncan be lavish and humble? What kind of imagery does she use in scene VI?
Lady Macbeth shows due deference to her monarch, and indeed he has been generous to her husband. By visiting their home, he is also showing gratitude and respect for his loyal subjects, and it would be appropriate for her to be deferential and reverent as tradition would require. She also follows a pattern which Banquo alluded to when he and Macbeth met the witches-
oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's
In deepest consequence—
Lady Macbeth speaks the truth to Duncan as he is indeed a benificent and generous monarch. She lies, however, about the gratitude she feels toward him.
What Lady Macbeth says to Duncan is pure deception, meant to flatter him and ensure that he does not suspect that there is anything for him to fear in her home. She feigns humblesness and fealty, all the while knowing Duncan will not live to see the dawn.
This is also a question of protocol. The way you greet a king is important. Anyone can't just go up to a king. There is a layer of respect in between. Royalty is supposed to be separated from the common folk and even other nobility.
Let us also remember the advice that Lady Macbeth has just given her husband, and which she shows she fulfills expertly herself. She tells her husband that he must:
...look like th'innocent flower,
But be the serpent under't.
This indicates the way in which Lady Macbeth uses lavish language to disguise her own intentions and evil ambitions, which of course works successfully with the poor, gullible Duncan.
As the liege of King Duncan, Lady Macbeth must show lavishness in her speech because she addressing her king, who, according to belief, was appointed over the country--and her--by God based on the principle of the divine right of kings. According to this belief, Lady Macbeth was speaking to a person second only to the triumvirate Godhead, therefore her most lavish and elegant language was required as a sign of reverence. [Incidentally, this is particularly significant when applied to her desperate plan for Macbeth's ascendancy: she was plotting for him to ascend to an appointment second only to God and ordained by God. This principle of the divine right of kings also puts a darker cast on the intervention of the dark magic of the witches.]
In Act One, scene six of Shakespeare's Macbeth, Lady Macbeth appropriately speaks to the King with respect and deference. This is expected because the King is of a higher station: all that serve the King are expected to act in accordance with their station. For example, the Macbeths are members of the nobility: they can address the King and entertain him at their home. Someone of lower rank, such as a simple soldier, would be able to do neither of these things.
Lady Macbeth accentuates this premise in lines 30-33. Here she uses imagery regarding an accounting ("audit") of what they have, but says that they only possess what they do because the King has granted it to them in the first place ("compt" meaning "held in trust"), and it is their obligation and/or duty to return all to him as he needs it. In other words, what they have is only theirs on loan—to be returned to the King when he requires it—without a second thought on his subjects' part. This promotes the sense of allegiance and respect that is fitting of Lady Macbeth when she welcomes the King of Scotland to her home.