Twelfth Night: Who states, "She pined in thought; /And, with a green and yellow melancholy, / She sat like Patience on a monument, / Smiling at grief" About whom?

The quote: "She pined in thought; /And, with a green and yellow melancholy, / She sat like Patience on a monument, / Smiling at grief" is spoken by Duke Orsino about Olivia in the play Twelfth Night by William Shakespeare.