Resistor for Green LEDs
If your power supply voltage is 12 volts DC, you can try using a resistor value close to 430 ohms. This assumes you only want to hook up one green LED (5mm type) in the circuit. The resistor should not dissipate more than 180mW so a standard 1/4 watt, 5%, carbon film resistor should work.
Green LEDs that contain a single die will tend to have a forward voltage drop between 3.0 to 3.6 volts. This also applies for white and blue LEDs. The exact voltage will depend on drive current and junction temperature. Before you can calculate an exact resistor value, add up the forward voltage of all series green LEDs in the circuit. Then subtract the total voltage drop from your power supply voltage, and divide by your desired drive current.
The exact type of resistor used in a circuit to drive green LEDs can vary. In most cases a cheap carbon film type will do. For surface mount applications a thick film type may work well. Keep in mind that resistors work fairly well for driving green LEDs that are 5mm type, at 20mA or less. Resistors do not work as well in circuits with high power LEDs. This is true for several reasons, but is mostly due to the fact that the resistor would need to be extremely large in order to dissipate the heat generated from the high current flowing through it.