Like their smell, the appearance of preparations is perceived individually as pleasant or unpleasant and is therefore often corrected or emphasised – especially in food and cosmetics. Colours make products appear more appealing, evoke associations and are therefore used not only in make-up.
Excipients
Colourants are coloured substances that are soluble in the water or oil phase of products, whereas pigments are insoluble solids that exist as suspensions. Both belong to the category of excipients and can be of plant, animal, synthetic or mineral origin. They do not contribute to the physiological effect of cosmetics.
Pigments
The most common mineral pigments are titanium dioxide (white)1, 2 and iron oxides (red, brown, black). Their colour effect results from the electronic constitution of metal atoms – including titanium, iron, chromium and manganese. It should be noted that the underlying metals may sometimes only be used in certain oxidation states – for example, chromium in its trivalent form (Cr3+); the corresponding chromium oxide is green. Hexavalent chromium (Cr6+) is carcinogenic and occurs, for example, in lead dichromate (yellow).
Depending on the composition of a cosmetic product, iron ions from iron oxides, for example, can migrate into the product in trace amounts and catalyse the auto-oxidation of sensitive components by atmospheric oxygen. Pigment particles are therefore often coated with aluminium hydroxide, silicon dioxide or similar substances. This technique can also be used to create colour nuances.
Dyes
Organic dyes and pigments contain so-called chromophores, i.e. planar molecular structures (π-electron systems) that absorb parts of the visible light spectrum. The best known are synthetic azo dyes, whose syllable "azo" refers to the nitrogen (N) they contain and "diazo" to the –N═N– group. These dyes, which are generally intense and inexpensive, have the disadvantage that they can be reductively split by bodily enzymes, reforming the components used in their synthesis. These include carcinogenic aromatic amines, among others. As a result, restrictions on their use in food, cosmetics and textiles are increasing.
An example of a different chemical structure is quinoline yellow, which is used as E 104 ("E number" for food additives) and CI 47005 (CI: Colour Index) as a hair dye.
Animal dyes are CI 75470 (carminic acid from scale insects; red) and CI 75800 (violet from purple snails).
Colouring agents
Many natural dyes are produced synthetically and are then considered nature-identical. Some also have active ingredient properties (table).
| Occurrence |
Colouring in solution |
Coloured component |
Effect |
| Carrots, tomatoes, peppers |
Yellow-red |
Carotenoids such as beta-carotene (provitamin A [E 160a]), lutein (E 161b), lycopene (E 160d), capsanthin (E 160c) |
Antioxidants, metabolism cf. vitamin A and retinoids |
| Green algae, crayfish, salmon |
Orange-red |
Astaxanthins (stereoisomers) |
Astaxanthins belong to the carotenoids, have antioxidant effects and are metabolised in the same way. Examples: astaxanthin (E 161j), canthaxanthin (E 161g) |
| Calendula extract |
Yellow |
Quercetin (flavonol) |
Polyphenolic antioxidant |
| Mitochondria, offal, nuts |
Yellow-orange |
Coenzyme Q10 (ubiquinone) |
Only the colourless, reduced coenzyme Q10 (ubiquinol) is a powerful antioxidant |
| Chamomile (essential oil) |
Blue |
Substituted azulenes |
Anti-inflammatory |
| Turmeric |
Yellow |
Curcumin (E 100) |
Anti-inflammatory effect suspected, but not confirmed |
| Fruit |
Red-brown |
Anthocyanins (E 163) |
Food colourings. Like the related oligomeric proanthocyanidins (OPC) in grape seed extract, they are powerful antioxidants. |
| Blue-green algae |
Blue suspension |
Phycocyanin |
Antioxidant |
| Zinc oxide – mineral |
White suspension |
Synthetic zinc oxide |
Antiseptic |
Most colourants are used in dissolved form, for example in lotions, in low concentrations, so that staining of textiles is largely impossible. The situation is different with pigments in lipsticks, powders and other make-up products: high concentrations are necessary to achieve sufficient coverage.
In concealer sticks, the colour green, which is necessary for the complementary neutralisation of reddish skin anomalies, is achieved by mixing black (CI 77499) and yellow (CI 77492) iron oxides. Zinc oxide and titanium dioxide enhance the covering effect.
Tattoos
As with the pigments used in decorative cosmetics, many heavy metal compounds, such as cadmium sulphide (yellow), have been banned over time in tattoos. They are on the list of prohibited substances (Annex II) of the Cosmetics Regulation (KVO). The approved pigments are listed in Annex IV. These include the aforementioned iron oxides with their different shades, titanium dioxide (white), chromium oxide (green) and carbon black (soot).
The organic dyes also listed in Annex IV cover a wide range of different substances, including quinacridones (CI 73900 red-violet), phthalocyanines (green-blue), diazo compounds (yellow-orange), diketopyrrolopyrroles (orange-red), indanthrone (blue) and dioxazines (violet), to name but a few.3
In tattoos, the stability of the dyes plays an even greater role than in cosmetics. However, degradation products formed in the epidermis are ultimately unavoidable. They are caused by:
- Reductive cleavage, e.g. of diazo compounds by enzymes in the epidermis and microbiome
- Oxidative attack by ROS (reactive oxygen species), i.e. reactive, endogenous oxygen compounds4
- Cleavage by the high-energy UV radiation of sunlight
Accordingly, a wide variety of tattoo fragments can be found, such as aniline, benzoic acid and naphthoic acid amides, benzo and naphthonitrile.
Analogous to unintentional degradation, the removal of tattoo colour particles is usually carried out with high-energy lasers or surgically.5 The chemical variant involves the use of reactive oxygen compounds such as hydrogen peroxide (stabilised)6 in combination with AHA acids (e.g. lactic acid) and complex-forming additives – supported by dermabrasion or other ablative procedures if necessary.
Brown through self-tanning
Dihydroxyacetone (DHA) reacts with the amino groups of the corneocytes. This condensation reaction leads to self-tanning – also known as melanoidin, which is similar to natural melanin but does not offer any protection against UV radiation. In addition to DHA, erythrulose is used, sometimes in combination with DHA.7 Furthermore, tanning accelerators – including tyrosine and derivatives – are commercially available and are intended to increase melanin production.
References
- H. Lautenschläger, Titandioxid – der Weißmacher, Beauty Forum medical 2022 (3), 16-17
- H. Lautenschläger, Ist Titandioxid alternativlos? Beauty Forum 2023 (3), 96-98
- S. Kochs, U. Hauri, M. Foerster, W. Bäumler, I. Schreiver, Neue Erkenntnisse über prähistorische Kunst, Chemie in unserer Zeit 58 (2), 80-92 (2024)
- H. Lautenschläger, Feuer und Feuerwehr – wie freie Radikale und Antioxidantien wirken, Medical Beauty Forum 2019 (2), 20-23
- H. Petersen, K. Roth, To Tattoo or not to Tattoo, Chemie in unserer Zeit 50 (1), 44-66 (2016)
- Fagron Group B.V., Patent specification WO 2008/128514 A1 dated 30 October 2008
- H. Lautenschläger, Selbstbräuner – Schön braun ohne Sonne, Kosmetische Praxis 2007 (6), 8-10
Dr Hans Lautenschläger |
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