]Yes, it is quite a dramatic moment when Wickham and Darcy meet in the road, and JA's decision not to tell more (including whose face turned what color)was brilliant. It creates great tension within the reader, who immediately wants to know more, please.

...absolutely. You really can't assign which was which at this point. To do so would be using information that comes after the moment. Why would we assume Darcy to be the angry one at this point. We've just met Wickham. We know almost nothing about him and nothing about a relationship of any kind between the two. But it is an obvious foreshadowing that there is a relationship and it's not a good one. After, Wickham's talk with Lizzie, then, yes, I would belive Darcy to be the one who turned red. As you say it was a stroke of brilliance to give us this much but withhold, which was which. Inquiring minds want to know? What's going on here? Definite tension.