A new fastness formula based on the CIEDE2000 color-difference formula was developed by B. Rigg and his coworkers. It is much simpler to calculate the staining fastness grade than the ISO 105-A04 fastness formula based on the CIELAB color-difference formula. Sample pair sets, which cover a wide color space range were accumulated from the NCS(Natural Color System) color book. for those sample pair sets, a visual measurement experiment and an instrumental measurement experiment of fastness grade were carried out. Each performance of the ISO 105-A43 fastness formula and newly developed fastness formula was compared through degree of agreement for visual measurement results. The newly developed fastness formula indicated improved performance for measuring fastness grade as it was confirmed that the performance of the current ISO fastness formula ISO 105-A04 for assessing staining, was inadequate for measuring fastness grade. Then the fastness formulae were examined more closely according to the particular color spaces and the correlation of hue, lightness and chroma for measuring staining fastness grade was also considered to recommend more improved fastness formula. By modifying the weighting functions of CIEDE2000, which is a basis of new fastness formula developed by B. Rigg, a modified fastness formula is proposed in this study.