## 1 Introduction  
Esthetic considerations have long influenced the development of restorative materials that closely mimic the optical properties of natural teeth. Traditional feldspathic ceramics were among the first materials used for this purpose. However, their limited mechanical strength restricted their use as stand-alone restorations, necessitating their combination with metal frameworks to improve durability. As a result, extensive research has been dedicated to enhancing the bond between veneering ceramics and the metal alloys used in these substructures [1,2]. Metal-ceramic restorations have been widely used in fixed prosthodonics for several decades, offering a reliable combination of mechanical strength and long-term durability. However, the presence of metal can hinder light transmission, leading to a less natural, opaque appearance. Moreover, the visibility of the metallic margin--especially through translucent surrounding tissues--poses a significant esthetic limitation [3,4]. Technological advancements have brought substantial progress, particularly with the introduction of CAD-CAM (computer-aided design and computer-aided manufacturing) ceramic restorations in the late 1980s. This innovation enabled the standardization of material quality, reduced production costs, and improved predictability throughout the manufacturing process. In the 1990s, yttria-stabilized zirconia ceramics were introduced, offering excellent mechanical properties and biocompatibility. However, their inherently opaque, whitish appearance presented an esthetic challenge, necessitating the application of a veneering ceramic layer. Despite enhancing esthetics, this veneering layer is prone to chipping, characterized by fractures in the outer ceramic that expose the underlying zirconia core, which typically remains intact [5,6]. To address this limitation, monolithic zirconia restorations were developed. Composed of highly crystalline ceramics, these materials exhibit reduced translucency, which can compromise esthetic outcomes. Nevertheless, their high flexural strength makes them well-suited for use in posterior regions, where mechanical performance is prioritized over esthetic considerations [7,8]. Pre-colored zirconia ceramics have been introduced to offer a combination of high mechanical strength and enhanced esthetics. Their increased translucency enables their application in both anterior and posterior restorations. A major advantage of these materials lies in the consistent color achieved during manufacturing, eliminating the variability associated with conventional infiltration or staining techniques, which are highly operator-dependent. Studies, such as that by [PERSON] et al. [9], have demonstrated that applying metallic oxides to sintered zirconia results in weak adhesion between the pigment and the zirconia surface. Additionally, the immersion technique can produce uneven coloration due to the limited penetration of metal ions. Color perception can be evaluated qualitatively through subjective visual comparison or quantitatively using electronic measurement devices. Portable spectrophotometers Vita Easyshade(r) Compact V are widely used for shade-matching research, and shade measurement reliability and accuracy have been supported by several studies. Measures of tooth or restoration color and color differences (\(\Delta\)E) can be made, allowing researchers to establish perceptibility and acceptability thresholds [10-15]. In color science, color differences are measured using the \(\Delta\)E formula, giving a quantitative representation of the perceived difference between paired colored specimens measured under set experimental conditions. The smallest perceptible color difference or perceptibility threshold refers to the smallest color difference detected by the human eye, while the highest acceptable or acceptability threshold refers to the largest color difference accepted by 50% of the observers as an imperceptible color difference [12].