OBJECTIVE: The colour of a nail polish varies according to the nail on which it is applied. The objective of this study was to predict the colour of the nail polish on a given nail and to study how the colour varies depending on the nail polish thickness. METHODS: Six nail polishes were applied in one, two and three layers on the nails of one subject, thus forming eighteen samples. The spectral reflectances of the eighteen nail polishes applied on the nails with different thicknesses were obtained by spectrophotometry. The spectral reflectances of the nails without polish were also measured using the same technique. The thicknesses of nail polishes were measured by high-definition optical coherence tomography (HD-OCT). Then, to determine the physical parameters of the nail polish itself, we applied the six nail polishes on an opacity drawdown chart and we measured the spectral reflectance and the thickness of each patch using spectrophotometry and HD-OCT, respectively. The Kubelka-Munk theory was used to get the predicted spectral reflectance of the nail polish applied on the nail according to the polish thickness by knowing the parameter of the polish itself and the spectral reflectance of the nail. The predicted spectral reflectances were finally compared with those measured directly on the nails. RESULTS: The predicted spectral reflectances were rather close to measured ones. Consequently, knowing the colour of the nail without polish and the optical parameters of the nail polish itself, we can estimate the colour of the nail polish applied on the nail depending on its thickness. CONCLUSION: Our study showed that the Kubelka-Munk theory can be used to predict the nail polish colour. The ability to predict the real colour of a nail polish applied on a nail could help a nail polish manufacturer to improve his polish formulae in order to obtain a precise colour.R esum e OBJECTIF: La couleur d'un vernis a ongles peut varier selon l'ongle sur lequel celui-ci est appliqu e. L'objectif de cette etude etait de pr edire la couleur d'un vernis a ongles lorsque ce dernier est appliqu e sur un ongle donn e et d' etudier comment la couleur varie en fonction de l' epaisseur du vernis. METHODES: Six vernis a ongles ont et e d epos es en une, deux et trois couches sur les ongles d'un individu, formant ainsi dix-huit echantillons. Les r eflectances spectrales des dix-huit couches de vernis a ongles d' epaisseurs diff erentes appliqu ees sur les ongles ont et e obtenues par spectrophotom etrie. Les r eflectances spectrales des ongles sans vernis ont egalement et e mesur ees par la même technique. Les epaisseurs des vernis a ongles ont et e mesur ees par tomographie par coh erence optique haute d efinition (HD-OCT). Ensuite, afin d'acqu erir les param etres physiques propres du vernis a ongles, nous avons appliqu e les six vernis a ongles sur un papier diffusant et nous avons mesur e la r eflectance spectrale et l' epaisseur de chaque echantillon en utilisant respectivement un spectrophotom etre et un HD-OCT. Les coule...