Organic Light Emitting diode (OLED) displays are very widely spread: smartphones, TVs, cars, virtual reality… Like in in any other displays OLED displays require three primary colours red, green and blue. Then the mixing of those colours can give any desirable colour or shade.
"O" in OLED stands for organic. In commercial OLEDs organic layers are evaporated in between two electrodes to which the voltage is applied in order to inject current or charges into those organic layers. Then charges recombine to create light in light emitting red, green, blue layers.