A first sub-pixel, a second sub-pixel and a third sub-pixel of a pixel are utilized to display a first white image, then a first brightness of the first sub-pixel and a first brightness of the second sub-pixel are measured. The first sub-pixel, the second sub-pixel and a fourth sub-pixel of the pixel are utilized to display a second white image, then a second brightness of the first sub-pixel and a second brightness of the second sub-pixel are measured. How image data should be displayed is determined according to whether the first brightness of the first sub-pixel is greater than the second brightness of the first sub-pixel, whether the first brightness of the second sub-pixel is greater than the second brightness of the second sub-pixel, and whether a chromaticity coordinate of the image data is within a chromaticity range capable of being displayed by the first, second and third sub-pixels.