I believe alien units spawn with an immutable aggression level towards specific players (ie. 100% aggression for Player 1, 25% aggression for Player 2 etc for a specific alien unit); because while you attack the aliens or destroy their nests, you can see how their aggressiveness does not change, only new units will have different values.
That would explain the different aggression levels, but at 18:20 of the 2nd ARC video he destroyed a nest and an orange manticore became red.