Green wire has something to do with oil pressure -
Your water temp is over on the opposite side - red and yellow wire!
To test your gauge - put a piece of wire into the bullet connection and touch to earth - your gauge should move all the way to hot!
If none of your pipes are cold, it could be the thermostat not opening... take the thermostat out, place it in a pot of water and start to boil it - you should see the stat opening... that means your stat isn't faulty!