For both Macbeth and Lady Macbeth, blood is representative of their guilt. Immediately after he commits the murder of Duncan, Macbeth asks,
Will all great Neptune's ocean wash this blood
Clean from my hand? No, this my hand will rather
The multitudinous seas incarnadine
Making the green one red. (2.2.78-81)
Obviously, not even all the blood from a single man could turn the ocean from green to red, but Macbeth's guilt for killing Duncan, his king and kinsman, is so extreme that he uses hyperbole, or overstatement, to express how truly awful he feels.
Lady Macbeth, when she returns from smearing Duncan's chamberlains with his blood, says,
My hands are of your color, but I shame
To wear a heart so white. (2.2.82-83)
Her hands are covered in blood now, but she says that she would be embarrassed to be as guilt-ridden (and perhaps even cowardly) as he.
Later in the play, however, she sings a different tune. In Lady Macbeth's sleepwalking scene, she washes her hands over and over, and she cries,
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