Read the excerpt from act 4, scene 3 of The Tragedy of Macbeth. Macduff is discussing the possibility of Malcom taking the throne of Scotland from Macbeth.

Malcolm. It is myself I mean: in whom I know
All the particulars of vice so grafted
That, when they shall be open’d, black Macbeth
Will seem as pure as snow, and the poor state
Esteem him as a lamb, being compared
With my confineless harms.

What is the intended effect of the simile "That, when they shall be open’d, black Macbeth / Will seem as pure as snow”? It emphasizes that Macbeth is not really the villain he seems to be. It suggests that Malcolm’s secrets would make Macbeth seem innocent. It implies that Macbeth has secrets about Malcolm that he might share. It suggests that people would trust Macbeth if they were to know the truth.