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Wood colour metrics are increasingly being used in wood technology and ecology studies. Researchers usually determine the colours of the wood after treatment or in different habitats. There is very little research dedicated to the problem of colour variations among one specific species harvested in different forests or regions. The main purpose of the current research is to reveal and estimate the colour variability of oak species. For this study, a total of 89 samples were taken from the heartwood of seven oak species (<i>Quercus robur</i> L., <i>Quercus cerris</i> L., <i>Quercus rubra</i> L., <i>Quercus pedunculiflora</i> K. Koch., <i>Quercus pubescens</i> Willd., <i>Quercus petraea</i> (Matt.) Liebl., and <i>Quercus palustris</i> Muenchh.). The CIELAB system was used for the assessment of the colour differences. To determine the colour groups and variations, K-means clustering was used. The results show that colour variations do exist. According to the cluster analysis, at least five types of oak wood can be distinguished (because in some clusters, very few samples were present) in the investigated forests. The differences are mainly observed in terms of the lightness (<i>L*</i>) and yellowness (<i>b*</i>). Redness is not a feature by which oak wood differs, but the wood can be brighter or more yellow in some of the samples. The density of the Romanian oaks in the sample does not influence the colour coordinates. The only coordinate affected is <i>a*</i>, but with a very small probability.