The first LEDs were red, and they got their start as indicator lights for circuit boards and small electrical equipment for this reason.
LEDs consist of layers of semiconductor crystals stacked on a wafer. As layers are added, dopants are incorporated to determine the color of the LED. Next, the tiny wafer is placed into molten liquid, and metal contacts and leads are added. The mixture used in the first LEDs produced a natural red color. Today, new processes make it possible to deliver a variety of colors. (Wired)
One of the next stages in LED history featured the development of high-brightness LEDs. As LEDs were developed, the light levels increased to the extent that they could be considered for applications outside simple indicator lamps. By 1987, AlGaAs (aluminium gallium arsenide) diodes were bright enough for the first applications within lighting. (Electronics Notes)
The first applications for these diodes was within the automotive industry, where red LEDs were used for vehicle brake lights and traffic lights. Here, LEDs were of particular interest, because they offered increased reliability over the incandescent lights that had been previously used. (Electronics Notes)