Color-adjustable light sources facilitate both mood lighting and daylight harvesting. A single duty cycle can be used by a bi-color LED to adjust the correlated color temperature by associating it with the duty cycle of the pulse width modulation dimming signal of the cold and warm light sources. The one-to-one mapping relationship between the single duty cycle and the correlated color temperature is based on the color mixing theory of bi-color LEDs. A method to correlate the dimming signals for cold and warm LEDs is presented. The influence of the time characteristics of the two basic signals on dimming and color temperature adjustment is analyzed. The dimming system of bi-color LEDs is designed, and the method used to adjust the correlated color temperature with a single duty cycle is verified. The experiment showed that the correlated color temperature can be accurately adjusted by the proposed method.