Whoever shames his friend in public to the point of making him turn pale is as if he sheds blood...for we see that the red drains out of his face and is replaced by white. [Ben Ish Chai]
A pious man was once insulted in the synagogue. When he came home, he sent the insulter a basket of grapes as a gift, with the following message: "You have presented me today with a basketful of your mitzvot. I, too, present you with a laden basket".
Why, if Reuven insults and embarrasses Shimon, do Reuven's mitzvot go to Shimon and Shimon's sins go to Reuven?
Red represents sins, and white represents mitzvot, as in: "Though your sins be as scarlet, they will become white as snow" [Isaiah 1:18]. When Reuven shames Shimon, he replaces the red in Shimon's face with white. Measure for measure, the red of Shimon's sins will replace the white of mitzvot in Reuven's soul.
Source: from the writings of the Ben Ish Chai