It was a clear red according to the rule book but the rule is harsh and there would have been quite a few players who would have kept running instead of looking for that red and then I'm not sure anything would have happened.
Ultimately its a poor decision by Dunk though. But shit happens...
I think the red was a bit harsh but obviously its a poor decision from Dunk and he knew it immediatly. Shit happens though, better defenders than him have made worse decisions than that over the years. Forget and move on.