## 4 Discussion  
The study of color is fundamental in dental esthetics as the shade of a restoration is often one of the most critical factors influencing a patient's perception of its quality. Consequently, a comprehensive understanding of tooth color, restorative materials, and the dental cements used in clinical practice is essential for achieving optimal esthetic outcomes [15]. Although zirconia ceramics are widely available, pre-colored zirconia represents a more streamlined option; however, it remains relatively underexplored in the literature. Therefore, the aim of this study was to evaluate the color outcomes of restorations fabricated from pre-colored zirconia provided by two commercial brands, applied over three different substrate types using two distinct resin cements. According to studies by [PERSON] [16] and [PERSON] et al. [17], the final color of restorations is influenced by ceramic thickness, the translucency level of zirconia, the type of cement, and the underlying substrate [16; 17; 18]. To minimize the influence of variables such as differences in testing methods or material composition on the final restoration color, this study utilized pre-colored zirconia blocks from two distinct commercial brands. This approach ensured that the restoration color remained independent of operator technique. Furthermore, all color measurements were performed under consistent conditions--at the same location and with standardized illumination, despite the manufacturer's claims that ambient light does not affect the final color of a restoration [19]. To simulate various clinical scenarios, three substrate types were selected: metal and white zirconia, representing post-core rehabilitations and abutments for implant-supported restorations, and enamel/dentin, simulating a natural tooth preparation for a full monolithic crown. The results demonstrated that the \(\Delta\)E values were significantly influenced by the substrate, cement, and ceramic brand, thus rejecting the null hypotheses initially proposed. Although the relationship between thickness and translucency is perceptible, there is limited information regarding the translucency of different types of zirconia when prepared at clinically required thicknesses, especially for pre-shaded materials, making it difficult to select an appropriate thickness for a specific abutment color [13; 20]. In this study, the tested samples were produced with a uniform thickness of 1.5 mm, simulating the average thickness of a crown-type restoration determined in another study [11]. With a higher thickness for a monolithic restoration, the influence of the  
Figure 14: Box plots of the \(\Delta\)E values related to glycerol and the two cements used (glycerol vs. light, \(p=0.072\); glycerol vs. neutral, \(p=1.000\); neutral vs. light, \(p=0.032\)). substrate was expected to be null. However, even though the restorations had a constant thickness and a standardized A2 color, the obtained \(\Delta\)E values varied among the different substrates. Only the AG crowns over the zirconia substrate with interposed glycerol had an acceptable result (\(\Delta\)E = 1.35), close to imperceptibility and well below the clinical acceptability threshold limits [12]. Several in vivo tests detected the mean values of \(\Delta\)E\({}^{*}_{\text{ab}}\) = 1.6 for a color match between a natural tooth and an all-ceramic restoration and \(\Delta\)E\({}^{*}_{\text{ab}}\) < 3.7 as the most commonly used for the clinical acceptability threshold. The reference color used in this study for the brand materials was VITA A2, where the spectrophotometer analyzes color based on the VITA scale. However, there is a known lack of consistency between different ceramic material brands and shade guides [21]. Because the zirconia blocks used were not manufactured by VITA, the use of equipment based on VITA original colors may have influenced the study results.