How much resistance are needed when you connect the green light-emitting diode to 12V and 5V DC ? LED current is generally 5 ~ 10mA, and voltage drop 2V, so 12V input need 1K ~ 2K resistor, and 5V need 470 ~ 1K resistor.
Even high brightness LED generally not more than 25mA, unless it is the kind of 1W, 3W, 5W. Recently used high-brightness LED as the indicator, the brightness is enough in 10mA-20mA. Different working voltage decide different colors, usually between 2-3V, optional 2.5V.
Series resistance = (12-2.5) / 10 = 1kΩ to (12-2.5) / 20 = 470Ω. If you only reach the lighting purpose, does not consider the efficiency, calculated as follows:
R = (supply voltage - input voltage of the light-emitting diode) / light-emitting diode current
R = (220-2) / 20 = 10.9K Ohms.