Imagine a world in black and white. It is difficult, right? We love colours! It is in our clothes, it is in our homes, and it definitely is in our cosmetics. The range of colours we can find for lipsticks, eyeshadows and other cosmetics is endless, just as much as the possibilities they offer of having different makeup looks: colours allow us to express ourselves.
The responsibles of noticing the colours in the cosmetics are the colour additives included in their formulations. Colour additives can be classified into inorganic or organic: inorganic pigments are normally metal oxides, while organic pigments are carbon-based molecules. Normally, inorganic colorants are more resistant to heat and light, but organic ones have brighter colours (1). If we look at their solubility, colorants or dyes are soluble in water or in oil, while pigments are insoluble and remain as crystals or particles when are applied (2).
Different colorants combinations give different colours to the cosmetics.
Combinations of colorants must be carefully designed considering both the shades of the colours and the characteristics of the pigments: it is not easy at all to develop a product that will look great under any lighting and be suitable for the type of formulation (3). If you would like to test your ability to differentiate between different colour shades, here is a link you can check.
Recently, the so-called colour changing cosmetics have become quite popular. How is it possible that a lipstick that appears to be green becomes pink when applied on the lips? It certainly seems like magic if you are not familiarized with a specific kind of colorants called bromo acids. These are water insoluble dyes that change their colour according to the pH. In their formulation they remain hidden, but when applied in the skin, they would turn into a range of reddish colours. This is the reason why we can have lipsticks that appear to be transparent and when applied they colour our lips in pink: the colorant was actually there, but in its formulation’s pH the colour was transparent.
Colour changing lipsticks can have any colour, but when applied in the skin they turn to reddish colours thanks to the bromo acids reacting to the change of pH.
Another application of the bromo acids is the formulation of indelible lipsticks. Bromo acids have the interesting property of staining the lips and are often used in combination with pigments that have a similar shade, so that when the pigments wear off, the stain of the bromo acids is still there. However, not all are advantages: bromo acids can dry the skin, so they have to be combined with emollients (4).
But bromo acids are not the only way of developing a product that changes the colour when it is applied. Colorants can also be encapsulated in the formulation, so that they are released when applied by rubbing in the skin, showing their colour (5). In this case, the colour in the formulation is present (it is not transparent) but it is hidden by a tiny capsule that we cannot see.
Another type of product that changes its colour when applied in skin are the bronzing products containing dihydroxyacetone. This compound is absorbed by the stratum corneum (the outer layer of the skin) and interacts with proteins forming brownish coloured products called melanoidins, giving the skin a tanned appearance without the risk of photodamage (6). However, recently it has been pointed out that dihydroxyacetone can produce oxidative stress in skin cells, questioning whether it is an adequate product to colour our skin with (7).
When it comes to colours in cosmetics, knowledge of how to combine the different colour additives is essential to obtain the effect we want. Traditional colorants, combined with new innovative colouring agents, give us endless possibilities. Science is the key to understand the characteristics of traditional colorants and to develop colouring additives and technologies with better characteristics, brighter colours and colour changing effects. Because sometimes the intention is not only to colour our skin, but also to produce “magic” effects and turn cosmetics into something fun.
Bibliography
(1) The chemistry of cosmetics - Curious https://www.science.org.au/curious/people-medicine/chemistry-cosmetics (accessed Mar 10, 2020).
(2) Colouring Agents in Cosmetic Products (Excluding Hair Dyes): Types of Decorative Cosmetic Products. 2007, 141–152. https://doi.org/10.1016/B978-044452260-3/50031-0.
(3) Expression of Color in the EU https://www.cosmeticsandtoiletries.com/regulatory/region/europe/Expression-of-Color-in-the-EU--241837011.html (accessed Mar 10, 2020).
(4) Cosmetics and Skin: Indelible Lipsticks https://www.cosmeticsandskin.com/cdc/indelible.php (accessed Mar 10, 2020).
(5) Can cosmetics really change color to match your skin? https://thebeautybrains.com/2011/05/can-cosmetics-really-change-color-to-match-your-skin/ (accessed Mar 10, 2020).
(6) Huang, A.; Brody, N.; Liebman, T. N. Dihydroxyacetone and Sunless Tanning: Knowledge, Myths, and Current Understanding. Journal of the American Academy of Dermatology. Mosby Inc. November 1, 2017, pp 991–992. https://doi.org/10.1016/j.jaad.2017.04.1117.
(7) Smith, K. R.; Granberry, M.; Tan, M. C. B.; Daniel, C. L.; Gassman, N. R. Dihydroxyacetone Induces G2/M Arrest and Apoptotic Cell Death in A375P Melanoma Cells. Environ. Toxicol. 2018, 33 (3), 333–342. https://doi.org/10.1002/tox.22520.
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