Have you ever bought a product because of its color? The answer to this question is probably yes, especially when it comes to cosmetic products, as color is one of the factors that play a key role in promoting sales in this niche market . But you can ask yourself: “from where these colors come from?”  Their origin can be from synthetic or natural molecules, entitled pigments and dyes. Most dyes utilized in cosmetic formulations are synthetic due to their durability and low production cost [1,3].
Figure 1. Diversity of colors found on eye shadow makeup.
However, despite the beautiful colors ensured by dyes, colorants are usually sensitive to UV light, pH and oxidation or reduction reactions: this confers to these substances a dangerous side when applied to the human body, making them able to suffer chemical transformations and cause harmful effects in our bodies . It was proven that azo dyes can release aromatic amines, causing toxic, mutagenic, and carcinogenic effects in humans . Nonetheless, the fact that some dyes can penetrate the skin increases their toxicity, causing allergies and inflammation. Xanthene’s dyes, that usually confers a red color, are an example of colorants that can induce inflammatory processes and lead to the formation of rough skin .
For this reason, several dyes have regulatory restrictions, with their maximum concentration tabulated or usage restricted in specific products . There are even some strict cases, like Rhodamine B, that have been banned by the EU . Therefore, the need of legislations to control the presence of these substances in cosmetic products is from extreme relevance and it is done mainly by the European Regulation of Cosmetic Products (EC 1223/2009)  from the European Commission (EC) . However, despite all these concerns, currently, there is no evidence of a stablished and official analytical methodology to perform the safety control analysis of dyes in cosmetics .
It is important to stress that cosmetics are truly diverse in chemical and physical composition and that these characteristics must be considered when performing the dye analysis. The first step is the sample preparation, in which extraction methods are the most employed according to the literature [1,3] and are one of the most time and solvent consuming phase , revealing a trend for the development of more green methodologies. The aim of this step is to separate the dyes from the rest of the compounds that are found in the cosmetic formulation. A more detailed overview can be found in the Figure 2.
Figure 2. Sample preparation and most used extraction procedures in the analysis of dyes in cosmetics .
After the extraction, to qualify and quantify the present dyes in the cosmetic, it is required an analytical method. High Performance Liquid Chromatography (HPLC) is the main technique currently chosen [1,3], being coupled with a detector (equipment that detects the molecules) of Mass Spectrometry (MS), Ultraviolet-Visible (UV-Vis) or Fluorescence, which has a specific measurement regarding the chemical properties of the substances. These techniques are better explained in the Figure 3.
Figure 3. Sample analysis and most used HPLC detectors employed in the analysis of dyes in cosmetics .
Overall, although there are many available analytical techniques to perform the analysis of dyes in cosmetics, it is still evident the actual need of the development of standardized analytical methodologies for the regulatory and safety control of dyes in cosmetics. Hence, it can be seen both as a challenging and promising field of research!
 Guerra, E.; Llompart, M.; Garcia-Jares, C.. Analysis of Dyes in Cosmetics: Challenges and Recent Developments. Cosmetics 2018, 5(3), 47; https://doi.org/10.3390/cosmetics5030047 .
 Color changing cosmetics: seems magic, but it is science. Elixabet Lerga Ibarlucea. https://www.emotion-master-studentproject.eu/post/color-changing-cosmetics-seems-magic-but-it-is-science (Acessed on November 14, 2020).
 Lores, M.; Llompart, M.; Alvarez-Rivera, G.; Vila, M.; Celeiro, M.; Lamas, P.; Garcia-Jares, C.. Positive lists of cosmetic ingredients: Analytical methodology for regulatory and safety controls – A review. Analytica Chemica Acta 2016, 915, 1-26; https://doi.org/10.1016/j.aca.2016.02.033.
 REGULATION (EC) No 1223/2009 OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 30 November 2009 on cosmetic products. https://ec.europa.eu/health/sites/health/files/endocrine_disruptors/docs/cosmetic_1223_2009_regulation_en.pdf (Accessed on November 14, 2020).
 Skoog, D.A.; Holler, F.J.; Crouch, S.R.. Principles of Instrumental Analysis. Cengage Learning, 7th Edition, 2018, Boston - USA.