Abstract: A method for rapid and quantitative assay of fluid specimen is disclosed. This method utilizes the principle of primary colors: any color can be discomposed into three primary colors or multiple primary colors (for example, red, green and blue). The molecules of the chemical being measured are bound to three colored substrates at different binding constant. Thus different concentration of the chemical will result in different final colors chromatography imaging. This makes the quantitative assay possible. This method can also be used to detect qualitatively the existence of multiple chemicals simultaneously.