The size of perceptual difference of colors (j, k) is scaled as d(jk) by selecting a pair of Munsell grays in which the lightness difference matches in size with the color difference. Hence, d is given in terms of Munsell V. The degree of principal hue component a! in a color j is scaled as xi(alpha)(j) by making marks on a line segment and the range of xi(alpha) is from 0 to 10. By plotting xi(alpha)(H V/C) on Munsell H-circle, principal hue curves <(xi)over bar>(alpha)(H V/C) are defined, where alpha = R Y, G, B, V = 4-7, and C = 2-10. In this process, similar plots of NCS codes (c phi(alpha)) are used as references. The curves <(xi)over bar>(alpha)(H V/C) tell us the appearance of Munsell colors (H V/C) and also enable us to predict color differences. The relationship between d(jk) and Delta V = \Vj - V-k\, Delta<(xi)over bar>(alpha) = \<(xi)over bar>(alpha) = (H-j V-j/C-j) -<(xi)over bar>(alpha)(H-k V-k/C-k)\ is tested in various ways, e.g., logarithmic, power, Minkowski-type functions. The best predictor a is given by a simple linear form, (d) over tilde = a(v)Delta V + {d(0) + Sigma a(alpha)Delta<(xi)over bar>(alpha)} For 899 pairs (j,k), 706 differing in H, C and 193 differing in H, V, C, a(v) = 0.459, d(o) = 0.610, a(R) = 0.199, a(Y) = 0.031, a(G) = 0.098 a(B) = 0.136, and the root-mean-squares of (d(jk) - (d) over tilde(jk)) is 0.338 in the matched V-unit.