I don't have the expansion.
How do the red zombies work?
They are scenario zombies. Used in specific scenarios, including the scenario modification of Zombie Horde (21) which means there is a total of 21 zombies in the zombie pool than the typical 14. I also use them to add players to the game. For every zombie player there are 2 heroes, thus 3 zombie players and 3-6 human players makes for a very interesting game.