Luminous intensity (brightness), wavelength (hue or color), and forward voltage are the three main parameters of an LED that are affected by temperature.
When compared to room temperature, higher temperatures cause the luminous intensity to slightly decrease (dimmer), the wavelength slightly lengthens (shifts towards red spectrum), and the forward voltage slightly decreases. Colder temperatures cause the luminous intensity to increase (brighter), the wavelength slightly shortens (shifts towards blue spectrum), and the forward voltage slightly increases. The rate of change in wavelength is typically around 0.1 nm/°C. For forward voltage, it is typically around 2mV/°C.