Zombies are green, move very 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 human directly in front of it, it tries to kill them. They will succeed, unless the human population is low. In that case, there is a chance the human will expect the zombie, and have a chance to attack first!