Benvolio compares Rosaline to a crow. He is trying to get Romeo to forget about Rosaline.
Go thither, and with unattainted eye
Compare her face with some that I shall show,(90)
And I will make thee think thy swan a crow. (Act 1, Scene 2, p. 21)
Basically, Benvolio is suggesting that Rosaline is not as good as Romeo thinks she is. Romeo is pining over Rosaline because she dumped him. He is so depressed that Benvolio encourages him to go to the Capulet party, where Rosaline will be, and see how much prettier all the other girls are.
As we know, Romeo does find his “swan” at the Capulet party—he falls in love with Juliet. Juliet makes him instantly forget about Rosaline.
Romeo is highly sensitive, but also highly fickle and changeable. He is deeply in love with Rosaline, and then he forgets her instantly. Benvolio seems to make an appropriate suggestion!