An old dim 2.1V green LED has almost the same forward voltage as a red or yellow LED.
But a modern very bright green LED has the same 3.6V forward voltage as a blue or white LED.
The forward voltage might be from 3.2V to 3.6V.
COB LEDs have multiple smaller LEDs all in series on a Silicon substrate, and will have a much higher voltage than normal LEDs so it doesn't apply to those.
Bowden has a led calculator that allows you to put in any forward voltage. http://www.bowdenshobbycircuits.info/led.htm
For the COB and CREE leds, look at the datasheet for the actual forward voltage.
Usualy the COB and CREE leds are driven using a constant current source and not a powersupply with a resistor.