Introduction Walnut (Juglans regia L.) belongs to the family Juglandaceae (Şen, 2011). It ranks third in nut production worldwide after cashews and almonds (Amin et al., 2017). Walnuts are quite rich in oils, proteins, vitamins, fibers, minerals, and antioxidants; thus they are considered quite healthy nuts for human nutrition (Khir et al., 2014). Walnuts are used not only as an individual foodstuff, but also as an ingredient of several baked and processed foods (Eliseeva et al., 2017). The physical properties of agricultural products have long attracted the attention of researchers (Bahrami et al., 2017). Previous researchers assessed the quality attributes of agricultural products such as shape, size, density, ripening level, moisture and oil content, firmness, flavor, and color of the products (Titova et al., 2015). The food industry uses various products in quite different sizes, colors, and shapes. Color can be used as a significant parameter in designing mechanical equipment to be used in sorting and grading processes of agricultural products (Mohsenin 1984; Pathare et al., 2013; Mahawar et al., 2017). Quality standards for the nut and kernel of walnuts have already been established by the Turkish Standards Institute (TSE, 1990, 1991). Shell thickness, kernel size, kernel ratio, flavor, and kernel color are also indicated as significant quality attributes of walnuts (Warmund, 2008). The ones with symmetric or round shapes usually have higher market value than the others (Jun et al., 2017). Color is considered a significant quality indicator for both raw and processed agricultural products since it is the first item perceived by consumers. It has a great potential to attract the attention and perceptions of buyers (Ellis and Kok, 2017; Goñi and Salvadori, 2017). For instance, for walnuts, light colors are usually preferred by consumers (Fuentealba et al., 2017). The color of agricultural products is usually assessed through L*, a*, and b* values or CIELab color space (Goñi and Salvadori, 2017). CIELab color space is commonly used for comparing product colors (Rodríguez-Pulido et al., 2013). It is an international standard adopted by the Commission Internationale d'Eclairage (CIE) in 1976 for color measurements (León et al., 2006).