Stylistic and Rhetorical Devices

The contrast of Desdemona's whiteness to Othello's blackness (she is "a pearl" pressed against his "sooty bosom") represents an unnatural pairing of good and evil, to the openly racist characters. But to others, such as the Duke, Othello is "far more fair than black" in character, The Duke is basically saying that Othello is as good as a white person.  This demonstrates that light is associated with being good and dark with being evil or of "the devil", implying that 'black' people are evil or unmoral. This connects to, an ironic, contrast between Othello and Iago because Othello is 'fair' inside, yet even he acknowledges his skin colour "Haply, for I am black", whereas Iago is white but has by far the blackest soul.

The associations, of darkness and fairness as good and bad are continued in Iago's soliloquy "so shall I turn her virtue into [black] pitch"(2.3.338). Black pitch here stands for corruption of her pure 'white' virtue.  Another example is when Othello cries later "My name, that was as fresh as Dian's visage is now begrimed and black as mine own face!"(3.3.338)

There are more subtle uses of light versus dark imagery aimed towards the audience. For instance, most of the scenes in which Iago 'carries out his evil plan' are set at night, as well as other scenes of disaster. For example, Iago's meetings with Roderigo, all the actions carried out by Roderigo to assist Iago in carrying out his plans (waking Brabantio, getting Cassio drunk and into a fight, his attempted murder of Cassio), and of course Desdemona's murder. Othello seems aware of the suitability of darkness to his actions. He speaks of killing Desdemona in the darkness, "Put out the light, and then put out the light ". Iago is also aware of the "fittingness" of darkness to his deeds, saying early on in the play "Hell and night, shall bring this monstrous birth to the world's light." Here he is already setting dark against light.