## 4 Discussion  
RBCs are widely used in the field of dentistry as an aesthetic restorative material and for the fabrication of artificial teeth, in addition to recontouring of abtument teeth [41; 42; 43]. The prognosis of any aesthetic restorative material depends on its ability for shade matching and color stability; it was demonstrated that light-cured RBCs can be disclosed over time by either extrinsic or intrinsic discoloration, resulting in patient frustration and extra replacement expenses [11]. Extrinsic factors include surface roughness, staining agent, and contact time in coloring solutions; frequent consumption of coloring and acidic drinks, such as Coca-Cola and coffee, may affect the color stability of RBCs through absorption and adsorption of colorant agents into the resin matrix [7; 9; 44]. Additionally, the yellowing effect can develop because of the teeth's and the restorative material's permeability allowing the penetration of pigments and chromogenic substances into the daily dietary intake [45]. The selection of beverages was made based on the daily consumption frequency, and they have often been used in previous in-vitro studies [4; 8; 9]. Ingredients such as sugar or white creamer were not added to beverages since they might interfere with the discoloring effect of the beverages. Simulation of long-term exposure to beverage solutions was conducted by storing the specimens in these solutions throughout the study period [4; 46]. Color changes of dental materials are usually evaluated by colorimeters and spectrophotometers since they prevent subjective interpretations and permit the detection of minor color differences. The CIE L\({}^{*}\), a\({}^{*}\), b\({}^{*}\) color evaluation method is considered a proper tool to assess alterations in color in dental materials. The color change values (\(\Delta\)E) were  
Figure 1: Color changes of different specimens after immersion in different beverages (N0: control; Zr3: 3 wt.% of ZrO\({}_{2}\); Zr7: 7 wt.% of ZrO\({}_{2}\); Ti3: 3 wt.% of TiO\({}_{2}\); Ti7: 7 wt.% of TiO\({}_{2}\); Si3; 3 wt.% of SiO\({}_{2}\); and Si7; 7 wt.% of SiO\({}_{2}\)). converted into NBS values to assess whether the color change was in the range of clinically acceptable levels [27, 47]. Our results showed significant differences between the \(\Delta\)E values of several groups (\(p\leq 0.05\)) after immersion of specimens in different beverages, causing us to conclude that the color stability of light-cured RBCs was significantly improved by NP addition, so the null hypothesis of the current study was rejected. In the current study, the spectrophotometer results revealed that great color changes occurred in specimens immersed in coffee, followed by tea and then Coca-Cola, while mineral water recorded the lowest \(\Delta\)E values. The observed NBS values were out of the clinically accepted limits with coffee and tea with marked color changes; this finding is in accordance with previous studies demonstrating that more discoloration of RBC specimens was associated with coffee solution [30, 48, 49]. Additionally, it agrees with [PERSON] et al. 's and [PERSON] et al. 's studies since they reported that coffee and tea produced discoloration in RBCs more than Coca-Cola [4, 37]. This outcome may be attributed to the different polarities of yellow colorants contained in tea and coffee; the polarity degree determines the penetration extent into the RBCs. Less polar dyes, such as coffee, can easily enter into the polymer matrix by the processes of absorption and adsorption, leading to more staining. On the other hand, more polar dyes, such as tea, only impregnate the material's surface and can be removed easily, thus causing less staining [37]. In disagreement with these findings, many studies have reported that tea is the most staining solution and can produce discoloration more than coffee [50, 51].