Since Shakespeare chose not to show King Duncan being murdered in his bed, he wanted to emphasize the reality and horror of the deed by showing both Macbeth and his wife with bloody hands. The purpose of all the looking at hands, showing of hands, and talking about hands is to call the attention of the audience to all the blood. First Macbeth says:
What hands are here? Ha, they pluck out mine eyes!
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.
Then his wife replies directly to this statement:
My hands are of your color, but I shame
To wear a heart so white.
They are both making a display of their bloody hands in order to produce a strong emotional effect on the audience. Their hands are, in effect, proof of the commission of a terribly bloody murder as well as a proof of their guilty partnership.
When Lady Macbeth says, "...but I shame / To wear a heart so white," she means, "I would be ashamed to wear a heart so white," i.e., I would be ashamed to be such a coward. She is telling her husband to stop bemoaning the crime he has committed. She is continually manipulating him by questioning his courage and manhood. She knows that this is only the beginning. They will have to keep their nerve in the morning when the King's body is discovered and there is pandemonium throughout the castle.
It seems that both husband and wife are "suiting the action to the word," as Hamlet (speaking for Shakespeare) tells the actors in that play. Macbeth and his wife not only display their bloody hands but get more blood on themselves while speaking the above-quoted lines. Macbeth probably would drag his fingers across his brow and eyes as he said, "What hands are here? Ha, they pluck out mine eyes." The audience would not know for a few moments whether Macbeth had actually, like Oedipus, actually torn his eyes out. His fingers would leave trails of blood all the way down his cheeks. Then when Lady Macbeth countered that she would "shame to wear a heart so white," she would wipe one hand across her white gown with the word "shame" and the other hand across the other side of the gown with the word "white."
Shakespeare must have wanted to use lots of blood in this scene to represent the terrible murder and to make up for the fact that he did not actually show the it being committed. He may have considered inserting a scene in which Macbeth stole into the King's bedchamber and stabbed the old man to death, but such a scene would obviously be hard to enact. And there would be no opportunity for Shakespeare's poetic dialogue. It would be a sort of dumb show and not emotionally effective like the scene in which Macbeth and his wife are smearing themselves with the King's blood. Both Macbeth and his wife are elsewhere given plenty of lines in which to describe what went on inside the King's bedchamber. For example:
There's one did laugh in's sleep, and one cried,
That they did wake each other: I stood and heard them:
But they did say their prayers and address'd them
Again to sleep.