Perhaps the most important thing to keep in mind is that the light output is proportional to the forward current. This means that trying to control the LED with the voltage is very difficult, as can be seen in this graph of forward current vs forward voltage. Once the LED is in the forward biased region, even a small change in the voltage results in a large change in the current. This then results in a significant change in the light output of the device. Color, light output and forward voltage change as a result of manufacturing processes.