 An LED's lens will often match the color of the light it emits, but that lens isn't what gives the LED its color. An LED's color is determined by the chemical composition of its internal semiconductor. Red and infrared LEDs often use aluminum gallium arsenide, yellow and green use gallium phosphide, and blue can be created from indium gallium nitride. Color is created by light traveling at different wavelengths, and creating these wavelengths requires differing amounts of energy, or voltage. Typically, red LEDs have a forward voltage of about 1.7, yellow 2.1 volts, green 2.2 volts, and blue and white LEDs operate at around 3.5 volts. So even if you don't have access to an LED's datasheet, you can make an educated guess about its forward voltage.