1. A display device, comprising: a color transformer configured to receive and transform 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 configured to calculate 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 is configured to calculate 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 is configured to respectively calculate a red image control signal, a green image control signal and a blue image control signal according to the gray level values and the decrement offsets, and the image controller is configured to calculate a white image control signal according to the gray level values and the compensation value of the white image; wherein one of the three-color image is zero and a signal value of the white image control signal is not zero.