The color of light emitted by an LED is determined primarily by...

The energy of the photon emitted by a light-emitting diode (LED) is approximately equal to the band gap energy $E_g$ of the semiconductor material used.

The wavelength $\lambda$ of the emitted light is related to the band gap energy by the equation:

$\lambda = \dfrac{hc}{E_g}$

Since the color of the emitted light depends on its wavelength $\lambda$, it is primarily determined by the band gap energy of the semiconductor material.

Answer: The band gap energy of the semiconductor material used.