## 4 Discussion
Nevertheless, the VITA shade guide remains the gold standard for color selection in dentistry, and companies should be consistent when using it for their color notation. The market offers various pre-colored monolithic zirconia shades, most of which align with the VITA Classical scale. According to [PERSON] et al. [22], color differences have been reported among different zirconia materials, as they exhibit slight dimensional and structural variations at the grain level, affecting light absorption and scattering properties. These variations, specific to the individual commercial formulation of each brand, may explain the observed differences [17; 22]. The two tested zirconia brands showed statistically significant differences, with the AG brand exhibiting lower \(\Delta\)E values. This difference could be attributed to two main factors: differences in the composition and variations in the manufacturer's coloring procedures. Regarding the composition, information is not disclosed by either manufacturer on their commercial websites or in brochures in the material packaging. However, small differences in yttrium and/or alumina content or even in the size of particles may be responsible for small changes in the optical behavior of zirconia [23]. The reduction of alumina content from 0.25 to 0.05 wt% is responsible for increasing translucency of the second generation of 3Y zirconia, and its content is approximately >70% of tetragonal and <30% of cubic zirconia, depending on the sintering temperature. This information was not disclosed by the manufacturers to allow for a better interpretation of material behavior [24]. In terms of zirconia coloring, only the ZZ brand provides details on how the material is colored to achieve the desired shade. Coloring substances are added to the zirconia powder during the early production stages, ensuring a more homogeneous and uniform coloration throughout the material. [PERSON] also states that manual coloring is becoming obsolete, and this type of pre-colored zirconia aims to increase production efficiency while eliminating shade variations caused by different dental laboratory techniques, thus improving the reliability of the manufacturing process. Regarding sintering temperatures, these play a crucial role in defining the ceramic's properties, as they directly affect microstructure and crystalline phase formation [25]. In this study, the sintering cycles differed between the two zirconia brands, but were made according to the manufacturers' instructions. Both manufacturers have sintering furnaces with pre-programmed cycles that cannot be altered by the user. For the AG brand, the cycle was longer (10 h) but at a lower final temperature (1450 \({}^{\circ}\)C), whereas for the ZZ brand, the cycle was shorter (8 h) but reached a higher maximum temperature (1500 \({}^{\circ}\)C). However, there is disagreement in the literature regarding the effects of sintering temperature. [PERSON] et al. [26] suggested that higher sintering temperatures and longer sintering times enhance translucency, improving light transmission, as there is a direct correlation between temperature increase and grain size [26]. On the other hand, studies by [PERSON] et al. [27] and [PERSON] et al. [25] contradict this assumption, reporting that higher sintering temperatures resulted in larger grains, which are generally associated with lower translucency [25,27]. The results obtained in this study support [PERSON]'s findings, as the ZZ zirconia, which underwent higher sintering temperatures, exhibited higher \(\Delta\)E values. Higher sintering temperatures enhance translucency; however, increased translucency also results in a higher influence of the underlying substrate. In a restoration, one of the final steps in the dental laboratory is glazing and/or polishing. These procedures can alter the optical properties of the ceramic, as they affect surface gloss, roughness, and brightness [16]. According to [PERSON] et al.