1. A display device, comprising: a red sub-pixel; a green sub-pixel; a blue sub-pixel; a white sub-pixel; a color transformer receiving and transforming a three-color image into a four-color image, wherein the four-color image comprises a red image, a green image, a blue image and a white image; and an image controller calculating a decrement offset of the red image, a decrement offset of the green image, a decrement offset of the blue image and a compensation value of the white image; wherein when a value of the white image is zero, the image controller calculates a gray level value of the red image, a gray level value of the green image, and a gray level value of the blue image, the image controller respectively calculates a red image control signal for the red sub-pixel, a green image control signal for the green sub-pixel and a blue image control signal for the blue sub pixel according to the gray level values and the decrement offsets, and the image controller calculates a white image control signal for the white sub-pixel according to the gray level values and the compensation value of the white image; wherein the display device displays an image by controlling the red sub-pixel according to the red image control signal, controlling the green sub-pixel according to the green image control signal, controlling the blue sub-pixel according to the blue image control signal, and controlling the white sub pixel according to the white image control signal; and wherein one of the three-color image is zero and a signal value of the white image control signal is not zero.