Zombies are green, move slowly and change direction randomly and frequently unless they can see something moving in front of them, in which case they start walking towards it. After a while they get bored and wander randomly again.
If a zombie finds a survivor standing directly in front of it, it bites and infects them; the survivor immediately joins the ranks of the undead.
Survivors are pink and run, occasionally changing direction at random. If they see a zombie directly in front of them, they turn around and panic.
Panicked survivors are bright pink. If a survivor sees another panicked survivor, it starts panicking as well. A panicked survivor who has seen nothing to panic about for a while will calm down again.
Source code @ Github