The Helmholtz-Kohlrausch effect is related with the chromatic strengths of spectral colors suggested by R. M. Evans. An estimate of the chromatic strengths of spectral colors is given by using the color-matching functions and their combinations. The estimate of the chromatic strength corresponds to "chromaticness per unit luminance" at each spectral color. The derived chromatic-strength function is very similar to the Helmholtz-Kohlrausch effect and the zero-grayness-function by Evans for spectral colors.