A method for determining a cross-manufacturer hair colorant recommendation, comprising: • assigning a plurality of hair colorants to in each case one standard starting hair color of a plurality of standard starting hair colors, wherein the plurality of hair colorants comprises a plurality of hair colorants of known composition and a plurality of hair colorants of unknown composition; • coloring hair samples using the plurality of hair colorants, wherein each one of the hair colorants is colored on a hair sample with the assigned standard starting hair color; • colorimetrically measuring the plurality of hair samples; • determining, separately for each standard starting hair color, a color difference between each of the plurality of colors of the hair samples that were colored by means of the hair colorants with the unknown composition, and each of the colors of the hair samples that were colored by means of the plurality of hair colorants of known composition: • assigning each of the hair colorants with the unknown composition to one of the hair colorants with the known composition such that the color difference for the hair colorants assigned to one another is minimized; • determining a user starting hair color; • determining a desired hair color of the user; • determining a first recommended hair colorant, wherein the hair colorant from the plurality of hair colorants of known composition for which the least color difference between the coloring result and the desired hair color is expected during coloring of hair comprising the user starting hair color is determined as the first recommended hair colorant by means of predictive analytics; and • determining a second recommended hair colorant, wherein the second recommended hair colorant is the hair colorant of unknown composition assigned to the first recommended hair colorant. • assigning a plurality of hair colorants to in each case one standard starting hair color of a plurality of standard starting hair colors, wherein the plurality of hair colorants comprises a plurality of hair colorants of known composition and a plurality of hair colorants of unknown composition; • coloring hair samples using the plurality of hair colorants, wherein each one of the hair colorants is colored on a hair sample with the assigned standard starting hair color; • colorimetrically measuring the plurality of hair samples; • determining, separately for each standard starting hair color, a color difference between each of the plurality of colors of the hair samples that were colored by means of the hair colorants with the unknown composition, and each of the colors of the hair samples that were colored by means of the plurality of hair colorants of known composition: • assigning each of the hair colorants with the unknown composition to one of the hair colorants with the known composition such that the color difference for the hair colorants assigned to one another is minimized; • determining a user starting hair color; • determining a desired hair color of the user; • determining a first recommended hair colorant, wherein the hair colorant from the plurality of hair colorants of known composition for which the least color difference between the coloring result and the desired hair color is expected during coloring of hair comprising the user starting hair color is determined as the first recommended hair colorant by means of predictive analytics; and • determining a second recommended hair colorant, wherein the second recommended hair colorant is the hair colorant of unknown composition assigned to the first recommended hair colorant. <EOS>