I've been using a stepper motor that has 5 wires to it. Orange, black, red, yellow, and brown. Now I'm going to use another stepper motor but this one has 6 wires. Colors yellow, red, orange, black, green, and brown. What does the extra color green wire do?
It conducts current, like every other wire. 
Google for Stepper, preferably on Images, and look at the various pictures until you find one that looks like yours. Then look at the page it came from. If you are lucky it will contain a diagram of what colour is connected to which coil. Or maybe there is a number/model on your stepper - google that.
Basically Unipolar Stepper have 5 or 6 wires - on the 5 wire version the "middle" of the two coils goes to one wire, on the 6 wire version each "middle" is seperate. Use an Ohm metere and measure every combination of the wires. You can then work out how the wires connect to the coils.
You need to measure the resistance of the windings. You should find two separate windings. Each should have a center connection that is equal resistance to the "ends". Those center connections go to the power supply, the other ends go to the stepper driver..
More info here: http://arduino-info.wikispaces.com/StepperMotors
With 6 wires you have a bi phase motor of the type shown here:-
http://www.thebox.myzen.co.uk/Workshop/Motors_4.html
Although this type of motor can have 4 wires, in you case each wire has a center tap. If you connect the two center taps together you get the equivalent of your 5 wire motor described here:-
http://www.thebox.myzen.co.uk/Workshop/Motors_3.html