Clearly the colour red is significant throughout the play to represent the blood that Macbeth shed in committing the terrible crime of regicide and doing away with Duncan. This acts as a constant reminder of his guilt, and the quote highlighted in #3 clearly shows that even after the blood has been washed away, Macbeth's hands will remain stained for long after.
Among the most famous lines in Macbeth are the lines in which Macbeth feels so polluted with blood after having killed the king that he thinks he can never be clean again:
The superb final juxtaposes green -- a color usually associated with nature and with life -- with red (a color associated in this play with death and with bloodshed). Macbeth imagines that he is so corrupted by his crime that he will make all the oceans red if he merely sticks his hand into the sea.
The two main colors in Macbeth are red and white. Red symbolizes both the murders which take place in the play and the guilt which accompanies the guilty. White, on the other hand, is associated with innocence. Lady Macbeth illustrates the use of this color representing innocence through her references to mother's milk and kindness.