With the continuous pursuit of high brightness and low power consumption display technology, RGBW display technology has been attracting increasing attention in the world. Various kinds of displays based on this technology have been produced in the market. The key of this technology is signal mapping algorithm which converts RGB signal into RGBW signal without color distortion and compatible with different sub-pixel layouts. This paper, on the basis of five kinds of signal mapping algorithms, analyzes the mode of action of newly added white sub-pixel affects display color, and the display performance affected by four kinds of sub-pixel layouts; it proposes the corresponding condition of excellent signal mapping algorithm based on colorimetry and deduces the universal equation of excellent signal mapping algorithm and brightness factor which could measure the ability of enhancing brightness. The simulation experiment shows that the signal mapping algorithm which satisfies universal equation could be able to maintain the hue and saturation better. The signal mapping algorithm’s ability to enhance brightness is effectively characterized by brightness factor. In conclusion, the proposed universal equation can be used to evaluate existing signal mapping algorithms, and it provides theoretical references for the research of new signal mapping algorithm which could be compatible with different hardware parameters and sub-pixel layouts, promote the popularization of RGBW display technology.