Elizabeth mentions Wickham to Darcy while they are dancing at the Netherfield Ball. How does Darcy respond?
Darcy responds with some sarcasm and irritation. He notes that Wickham is quite charming, but that he might not be good at being around very long (as is evidenced when he left Darcy's sister and other women he manipulated). Darcy has good reason to be irritated and angry about Wickham due to Wickham's gambling away money Darcy's father had given him and because Wickham deceived his sister and left her heartbroken.
During the Netherfield Ball in Ch.18 Darcy wishes to dance with Elizabeth who agrees to do so very reluctantly. The irony of the present situation is that Darcy has become secretly attracted to Elizabeth who is disgusted with him because of his pride and arrogance. Elizabeth is not aware that Darcy has begun to be attracted to her, she thinks that he hates her because she is not as rich as he is.
When the dance begins Elizabeth begins to banter with him and tease him. She then remarks on what happened the other day when she and her sisters were on their way to Meryton in Ch.15:
On distinguishing the ladies of the group, the two gentlemen [Darcy and Bingley] came directly towards them, and began the usual civilities. Bingley was the principal spokesman, and Miss Bennet the principal object. He was then, he said, on his way to Longbourn on purpose to inquire after her. Mr.Darcy corroborated it with a bow, and was beginning to determine not to fix his eyes on Elizabeth when they were suddenly arrested by the sight of the stranger, and Elizabeth happening to see the countenance of both as they looked at each other, was all astonishment at the effect of the meeting. Both changed colour, one looked white, the other red. Mr.Wickham after a few moments, touched his hat -- a salutation which Mr. Darcy just deigned to return. What could be the meaning of it? -- It was impossible to imagine; it was impossible not to long to know.
In Ch.16 Wickham succeeds in poisoning Elizabeth's mind by telling all sorts of lies about Darcy. He tells Elizabeth that if not for Darcy he would have now become a very successful and respected clergyman:
Now at the Netherfield Ball in Ch.18 as soon as Elizabeth mentions Wickham, Darcy's whole manner changes and he becomes very cold and aloof,
The effect was immediate. A deeper shade of hauteur overspread his features,
Elizabeth mistakes Darcy's reaction to be a sign of his unreasonable hatred towards Wickham. The irony of the situation is that she doesn't know the truth that Wickham had tried to elope with Darcy's own sister Georgiana Darcy [Ch.35] and that is why he hates him. This is the reason why Darcy's face became red with anger and Wickham's red with embarrassment when they ran into one another in Ch,.15.