When a signal receiving unit receives red, green, and blue sub-pixels of an RGB pixel, a chromaticity weight generation unit generates a chromaticity weight according to the lowest gray level and the highest gray level of the red, green, and blue sub-pixels of the RGB pixel. Then, a gray level generation unit generates gray levels of red, green, blue, and white sub-pixels of an RGBW pixel according to the chromaticity weight, gray levels of the red, green, and blue sub-pixels of the RGB pixel, three chromaticity correction values and the lowest gray level. A RGBW display displays the RGBW pixel according to the gray levels of the red, green, blue, and white sub-pixels of the RGBW pixel.