Compound chosen from dyes of azomethine type comprising two pyrazolopyridine units of formulae and, the leuco forms, optical and geometrical isomers, and tautomers thereof, and also the addition salts thereof with an acid or a base and the solvates thereof, such as hydrates: in which formulae and: • Z 1 represents an oxygen atom or a group -N(R 6)-; • Z' 1 represents an oxygen atom or a group -N(R' 6)-; when Z 1 represents -N(R 6)- and/or Z' 1 represents -N(R' 6)- then R 1 and R 6 and/or R' 1 and R' 6, respectively, may form, together with the nitrogen atom to which they are attached, an optionally substituted, 5- to 8-membered, optionally cationic, saturated, unsaturated or aromatic heterocycle; • R 1, R' 1, R 6, and R' 6 each independently represent: - a hydrogen atom, - a C 1 -C 10 alkyl radical optionally interrupted with one or more non-adjacent heteroatoms, preferably oxygen, and/or optionally substituted, preferably with one or more groups chosen from i) hydroxyl, ii) optionally substituted, 5- to 8-membered, cationic or non-cationic, saturated, unsaturated or aromatic (hetero)cycle, iii) -N(R')R", iv) -N + R'R"R'" with R', R" and R'" each independently representing a C 1 -C 6 alkyl group; - an optionally substituted, 5- to 8-membered, cationic or non-cationic, saturated, unsaturated or aromatic (hetero)cyclic radical; • R 2, R 3, R 4, R 5, R' 2, R' 3, R' 4 and R' 5 each independently represent: - a hydrogen atom, - an optionally substituted C 1 -C 4 alkyl radical, - a group chosen from -NH 2, -N R 10, -N(R 11)R 12, OH and -OR 9, with R 9 and R 10 representing an optionally substituted, linear or branched C 1 -C 6 alkyl, R 11 and R 12, which may be identical or different, representing an optionally substituted, linear or branched C 1 -C 6 alkyl, it being possible for R 11 and R 12 to form, together with the nitrogen atom to which they are attached, a saturated, unsaturated or aromatic 5- to 8-membered heterocycle optionally containing one or more other heteroatoms or groups chosen from N, O, S, S 2 and C, the heterocycle being optionally substituted; • R 2, R 3, R 4, R 5, R' 2, R' 3, R' 4 and R' 5 may form, in pairs with adjacent radicals, an optionally substituted, saturated or unsaturated (hetero)cycle; it being understood that when the compound of formula or is positively charged, then it comprises as many anionic counterions as cationic charges to achieve the electrical neutrality of the molecule. • Z 1 represents an oxygen atom or a group -N(R 6)-; • Z' 1 represents an oxygen atom or a group -N(R' 6)-; when Z 1 represents -N(R 6)- and/or Z' 1 represents -N(R' 6)- then R 1 and R 6 and/or R' 1 and R' 6, respectively, may form, together with the nitrogen atom to which they are attached, an optionally substituted, 5- to 8-membered, optionally cationic, saturated, unsaturated or aromatic heterocycle; • R 1, R' 1, R 6, and R' 6 each independently represent: - a hydrogen atom, - a C 1 -C 10 alkyl radical optionally interrupted with one or more non-adjacent heteroatoms, preferably oxygen, and/or optionally substituted, preferably with one or more groups chosen from i) hydroxyl, ii) optionally substituted, 5- to 8-membered, cationic or non-cationic, saturated, unsaturated or aromatic (hetero)cycle, iii) -N(R')R", iv) -N + R'R"R'" with R', R" and R'" each independently representing a C 1 -C 6 alkyl group; - an optionally substituted, 5- to 8-membered, cationic or non-cationic, saturated, unsaturated or aromatic (hetero)cyclic radical; - a hydrogen atom, - a C 1 -C 10 alkyl radical optionally interrupted with one or more non-adjacent heteroatoms, preferably oxygen, and/or optionally substituted, preferably with one or more groups chosen from i) hydroxyl, ii) optionally substituted, 5- to 8-membered, cationic or non-cationic, saturated, unsaturated or aromatic (hetero)cycle, iii) -N(R')R", iv) -N + R'R"R'" with R', R" and R'" each independently representing a C 1 -C 6 alkyl group; - an optionally substituted, 5- to 8-membered, cationic or non-cationic, saturated, unsaturated or aromatic (hetero)cyclic radical; • R 2, R 3, R 4, R 5, R' 2, R' 3, R' 4 and R' 5 each independently represent: - a hydrogen atom, - an optionally substituted C 1 -C 4 alkyl radical, - a group chosen from -NH 2, -N R 10, -N(R 11)R 12, OH and -OR 9, with R 9 and R 10 representing an optionally substituted, linear or branched C 1 -C 6 alkyl, R 11 and R 12, which may be identical or different, representing an optionally substituted, linear or branched C 1 -C 6 alkyl, it being possible for R 11 and R 12 to form, together with the nitrogen atom to which they are attached, a saturated, unsaturated or aromatic 5- to 8-membered heterocycle optionally containing one or more other heteroatoms or groups chosen from N, O, S, S 2 and C, the heterocycle being optionally substituted; - a hydrogen atom, - an optionally substituted C 1 -C 4 alkyl radical, - a group chosen from -NH 2, -N R 10, -N(R 11)R 12, OH and -OR 9, with R 9 and R 10 representing an optionally substituted, linear or branched C 1 -C 6 alkyl, R 11 and R 12, which may be identical or different, representing an optionally substituted, linear or branched C 1 -C 6 alkyl, it being possible for R 11 and R 12 to form, together with the nitrogen atom to which they are attached, a saturated, unsaturated or aromatic 5- to 8-membered heterocycle optionally containing one or more other heteroatoms or groups chosen from N, O, S, S 2 and C, the heterocycle being optionally substituted; • R 2, R 3, R 4, R 5, R' 2, R' 3, R' 4 and R' 5 may form, in pairs with adjacent radicals, an optionally substituted, saturated or unsaturated (hetero)cycle;