... Even though the explanation anger-fear seems logical, I am inclined to think it was the other way around - Darcy was white and Wickham turned red - remember, Wickham doesn't really have any feelings of remorse so he may just be angry at Darcy for invading his teritory and spoiling his fun. On the other hand there is Darcy, who suddenly remembered the pains of his sister and he might also be jealous - he has some inclination towards Elizabeth at that time, so it could be that unexplainable pale start when you see someone you like pleasantly talking to another (that reason for the pale face being subconscious, naturally).