1. A data conversion unit in a display device comprising: a color analysis portion configured to determine presence of a pure color component in an input image by analyzing R (red), G (green) and B (blue) components of RGBW (red, green blue and white) data signal of an input image; and a brightness adjustment portion configured to adjust brightness of W (white) component in the RGBW data signal according to a hue of the pure color component present in the input image.