Antimatter blue - no contest. If we are talking about what color has the most heritage to the original bronco. Check out Leno's Bronco restoration. I am actually picking AM because it pays the most tribute to the original (with a white top).
Unless the question is more mindless in the sense of "which has the most general retro appearance" rather than "what is more retro as paying homage to the original bronco". In that case, probably cactus gray as its a light muted color popular in the 60/70s.