A control system includes a clock unit for providing a first clock signal, and a second clock signal having a frequency lower than that of the first clock signal. Three control units are coupled respectively to three light emitting diodes (LED) emitting respectively three different colors. Each control unit is operable based on the clock signals and a corresponding set of first and second reference values to output a driving pulse signal to a corresponding LED such that the corresponding LED is driven by the driving pulse signal to emit light during a corresponding one of first, second and third time periods of a control cycle of the control system.