1. A method for colouring hair comprising the steps of:
i) providing a first hair colouring composition and a second hair colouring composition, wherein the pH of the second hair colouring composition is lower than the pH of the first hair colouring composition;
ii) applying a portion or all of the first hair colouring composition to the hair;
iii) applying a portion or all of the second hair colouring composition to the hair,
wherein each the first and the second hair colouring compositions comprises one or more oxidizing agents and one or more compounds selected from the group consisting of oxidative dye precursors, alkalizing agents and mixtures thereof.
2. The method according to claim 1, wherein the method further comprises the step of:
iv) rinsing the hair.
3. The method according to claim 1, wherein the portion or all of the first hair colouring composition is applied to the hair roots and the portion or all of the second hair colouring composition is applied to the hair lengths and tips.
4. The method according to claim 1, wherein the second hair colouring composition produces on the hair a coloration such that the colour variation between hair coloured with the second hair colouring composition and hair coloured with the first hair colouring composition is greater than or equal to 1, as measured according to the colour variation test method.
5. The method according to claim 1, wherein the second hair colouring composition produces on the hair a coloration such that the colour lightness difference between hair coloured with the second hair colouring composition and hair coloured with the first hair colouring composition is greater than 0, as measured according to the colour variation test method.
6. The method according to claim 1, wherein the difference of pH between the first hair colouring composition and the second hair colouring composition is at least 0.25.
7. The method according to claim 1, wherein the pH of each of the first and second hair colouring compositions is from 3 to 11.
8. The method according to claim 2, wherein the method further comprises the steps of:
a) mixing a first dye composition with a first developer composition to obtain the first hair colouring composition, wherein the first dye composition comprises one or more oxidative dye precursors and/or one or more alkalizing agents and the first developer composition comprises one or more oxidizing agents;
b) mixing a second dye composition with a second developer composition to obtain the second hair colouring composition, wherein the second dye composition comprises one or more oxidative dye precursors and/or one or more alkalizing agents and the second developer composition comprises one or more oxidizing agents, wherein the pH of the second developer composition is lower than the pH of the first developer composition,
wherein step b) is performed between steps ii) and iii) and the method further comprises the steps of waiting for a time period of from 10 min to 40 min, which is performed between steps ii) and iii) and waiting for a time period of from 5 min to 20 min which is performed between steps iii) and iv).
9. The method according to claim 2, wherein the method further comprises the steps of:
a) mixing a first dye composition with a first developer composition to obtain the first hair colouring composition, wherein the first dye composition comprises one or more oxidative dye precursors and/or one or more alkalizing agents and the first developer composition comprises one or more oxidizing agents;
b) mixing a second dye composition with a second developer composition to obtain the second hair colouring composition, wherein the second dye composition comprises one or more oxidative dye precursors and/or one or more alkalizing agents and the second developer composition comprises one or more oxidizing agents, wherein the pH of the second developer composition is lower than the pH of the first developer composition,
wherein step b) is performed between steps ii) and iii) and step b) is performed immediately after step ii) and step iii) is performed immediately after step b) and the method further comprises the step of waiting for a time period of from 5 min to 40 min which is performed between steps iii) and iv).
10. The method according to claim 2, wherein:
the method further comprises the step of mixing a first dye composition with a first developer composition to obtain the first hair colouring composition, wherein the first dye composition comprises one or more oxidative dye precursors and/or one or more alkalizing agents and the first developer composition comprises one or more oxidizing agents and
in step ii), a first portion of the first hair colouring composition is applied to the hair and a second portion of the first hair colouring composition is retained and
the method further comprises the step of adding a pH reducing agent to the second portion of the first hair colouring composition to obtain the second hair colouring composition.
11. The method according to claim 10, wherein the step of adding a pH reducing agent to the second portion of the first hair colouring composition to obtain the second hair colouring composition is performed between steps ii) and iii) and the method further comprises the step of waiting for a time period of from 10 min to 40 min which is performed between steps ii) and iii) and waiting for a time period of from 5 min to 20 min which is performed between steps iii) and iv).
12. The method according to claim 10, wherein the step of adding a pH reducing agent to the second portion of the first hair colouring composition to obtain the second hair colouring composition is performed immediately after step ii) and immediately before step iii) and the method further comprises the step of waiting for a time period of from 5 min to 40 min, which is performed between steps iii) and iv).
13. The method according to claim 10, wherein the pH reducing agent is selected from the group consisting of acetic acid, acetyl mandelic acid, adipic acid, aluminum lactate, aluminum triformate, ammonium lactate, ammonium molybdate, ammonium nitrate, ammonium thiocyanate, ammonium vanadate, ascorbic acid, azelaic acid, babassu acid, bakuhan, benzilic acid, bismuth citrate, boric acid, calcium citrate, calcium dihydrogen phosphate, calcium phosphate, citric acid, diammonium citrate, dioleyl phosphate, disodium pyrophosphate, fumaric acid, galacturonic acid, glucoheptonic acid, glucuronic acid, glutaric acid, glycine, glycolic acid, glyoxylic acid, hydrobromic acid, hydrochloric acid, hydroxyethylpiperazine ethane sulfonic acid, isobutyric acid, lactic acid, lactobionic acid, magnesium glycinate, magnesium lactate, maleic acid, malic acid, malonic acid, maltobionic acid, metaphosphoric acid, monosodium citrate, mudstone powder, phenolsulfonphthalein, phenyl mercuric borate, phosphoric acid, phosphorus pentoxide, potassium bicarbonate, potassium biphthalate, potassium magnesium aspartate, potassium sodium tartrate, potassium tartrate, propane tricarboxylic acid, quinic acid, ribonic acid, sebacic acid, sodium aspartate, sodium bisulfate, sodium borate, sodium butoxyethoxy acetate, sodium calcium boron phosphate, sodium calcium copper phosphate, sodium calcium zinc phosphate, sodium citrate, sodium glycolate, sodium lactate, sodium phosphate, sodium succinate, succinic acid, sulfuric acid, tartaric acid, taurine, tea-hydroiodide, trisodium sulfosuccinate, triticum vulgare protein, triticum vulgare seed extract, uric acid, zinc hexametaphosphate and mixtures thereof.
14. The method according to claim 10, wherein the pH reducing agent is selected from the group consisting of citric acid, phosphoric acid, salicylic acid, etidronic acid, acetic acid, ascorbic acid, hydrochloric acid, sulfuric acid and mixtures thereof.
15. The method according to claim 10, wherein the pH reducing agent is provided as a solid or a liquid.
16. The method according to claim 10, wherein from 90% to 10% by weight of the first hair colouring composition is applied as a first portion to the hair.
17. The method according to claim 1, wherein the first and second hair colouring compositions are applied to the hair with a brush and bowl applicator or a container to which a nozzle or a separate applicator device such as a comb or a brush is attached.
18. A hair colouring kit comprising a first dye composition, a second dye composition, a first developer composition and a second developer composition, wherein each of the first and second dye compositions comprises one or more oxidative dye precursors and/or one or more alkalizing agents and each of the first and second developer compositions comprises one or more oxidizing agents, wherein the pH of the second developer composition is lower than the pH of the first developer composition.
19. A hair colouring kit comprising a first dye composition, a first developer composition and a pH reducing agent wherein the first dye composition comprises one or more oxidative dye precursors and/or one or more alkalizing agents and the first developer composition comprises one or more oxidizing agents and wherein the first dye composition, the first developer composition and the pH reducing agent are packaged separately.