The color of an LED is a function of the material used to make the junction. There are two main flavors used in visible light LED junctions:
Indium gallium nitride (InGaN) is used to make up the blue, white, true green, and UV types.
Aluminum gallium indium phosphide (AlGaInP or AlInGaP) is used to make the red, yellow, and orange types.
The type of material used also has an effect on the rated forward voltage of the LED. It’s useful to think of these two different types of diodes even if you can’t remember the exact material—the red, yellow, and orange types have a forward voltage around 2 Volts, while the blue, white, and true green types have a forward voltage around 3.3 Volts.