A method of predicting the result of a hair-color-modifying treatment on a sample of hair comprising hair shafts of different color-types, the hair being natural-gray hair comprising a mixture of natural-white shafts and natural-pigment-containing shafts, or previously dyed natural-gray hair comprising a mixture of shafts of first and second color-types that are respectively derived from the natural-white and the natural-pigment-containing shafts, the method comprising: a) for each of a plurality of distinct regions of the sample, measuring a region-specific spectrum of light scattered from hair material within the region, to obtain a plurality of measured region-specific spectra; b) deriving from the plurality of region-specific spectra first and second initial spectra, the first initial spectrum being either a single region-specific spectrum of light scattered by natural-white shafts or the average of multiple region-specific spectra of light scattered by natural-white shafts and the second initial spectrum being either a single region-specific spectrum of light scattered by natural-pigment-containing shafts or the average of multiple region-specific spectra of light scattered by natural-pigment-containing shafts; c) deriving from the plurality of region-specific spectra a hair-shaft color-heterogeneity parameter of the hair-sample indicative of the relative fractions of natural-white shafts and natural-pigment-containing shafts within the sample, d) computing first and second predicted post-treatment spectra from the first and second initial spectra by respectively predicting a transformation of the first and second initial spectra following subjecting the sample of hair to the hair-color-modifying treatment, and e) computing from the first and second predicted post-treatment spectra and the heterogeneity parameter a predicted post-treatment spectrum for the entire sample of hair after subjecting the sample to the hair-color-modifying treatment. a) for each of a plurality of distinct regions of the sample, measuring a region-specific spectrum of light scattered from hair material within the region, to obtain a plurality of measured region-specific spectra; b) deriving from the plurality of region-specific spectra first and second initial spectra, the first initial spectrum being either a single region-specific spectrum of light scattered by natural-white shafts or the average of multiple region-specific spectra of light scattered by natural-white shafts and the second initial spectrum being either a single region-specific spectrum of light scattered by natural-pigment-containing shafts or the average of multiple region-specific spectra of light scattered by natural-pigment-containing shafts; c) deriving from the plurality of region-specific spectra a hair-shaft color-heterogeneity parameter of the hair-sample indicative of the relative fractions of natural-white shafts and natural-pigment-containing shafts within the sample, d) computing first and second predicted post-treatment spectra from the first and second initial spectra by respectively predicting a transformation of the first and second initial spectra following subjecting the sample of hair to the hair-color-modifying treatment, and e) computing from the first and second predicted post-treatment spectra and the heterogeneity parameter a predicted post-treatment spectrum for the entire sample of hair after subjecting the sample to the hair-color-modifying treatment.