## 4 Discussion
Compared to coffee and tea (pH 5.0 and 6.1, respectively), the Coca-Cola solution did not cause greater discoloration, although it had a low pH (pH 2.7), which may affect the surface roughness of RBCs. The minor degree of color change in Coca-Cola could be attributed to the absence of yellow colorants in its ingredients and the presence of carbonated water, which buffers the acidity present, thus reducing surface integrity damage [46]. Several articles are in agreement with our results [37, 52, 53]. On the other hand, [PERSON] et al. showed that Coca-Cola can cause color changes in composite resin more than coffee and tea [54]. A slight color alteration was detected in specimens soaked in mineral water (NBS \(\leq\) 1), which can be attributed to the water's absence of colorant and neutral pH. This outcome is in accordance with many previous studies [8, 9, 38, 47]. On the other hand, the slight detected color changes may be due to softening of the polymer since water causes network swelling and loosening of the frictional forces between polymer chains [55]. Color changes are considered acceptable in in-vitro studies if (\(\Delta\)E) \(\leq\) 3.7, while they become visually detected and unacceptable when \(\Delta\)E \(>\) 3.7 [11, 23]. Regarding the present outcomes, composite specimens immersed in mineral water and Coca-Cola after 6 days presented clinically acceptable color changes, while those immersed in coffee and tea were clinically unacceptable, in agreement with the results of [PERSON] et al. [4]. According to the findings of the current study, addition of NPs decreases color changes. This outcome is in agreement with previous studies that reported that the optical properties of light-cured RBCs improved with NP addition [56, 57, 58]. Irrespective of NP type, low concentrations (3 wt.%) presented less \(\Delta\)E compared with high concentrations (7 wt.%). This finding may be attributed to increased density and reduced porosity due to the presence of inorganic nanofillers within the material, resulting in less stain absorption [59]. In agreement with the present findings, [PERSON] et al. showed better discoloration resistance in RBC artificial teeth with silanized SiO\({}_{2}\) filler soaked in different beverages, and this finding may be explained by SiO\({}_{2}\) potentially being able to improve the hydrolytic stability and decreased water sorption, leading to fewer color changes [43]. Furthermore, [PERSON] et al. showed the same results concerning the anti-staining effects of ZrO\({}_{2}\), TiO\({}_{2}\), and SiO\({}_{2}\) on heat-polymerized PMMA [19]. Light refraction and reflection at the interface between the matrix and filler are affected by the differences between the inorganic nanoparticles and organic matrix refractive indices. Thus, the greater the refractive index difference, the greater the nanocomposite opacity [25; 26]. In contrast to the present study findings, [PERSON] et al. reported high surface roughness of nanocomposites due to the irregular arrangement of inorganic NPs, resulting in easier staining effects over time [44]. The discrepancies between the results of this study and other articles [44; 50; 51; 54] may be explained by differences in experimental methodology, and types and concentrations of the tested NPs, and the types and concentrations of the tested beverages, as well as the usage of different commercial RBCs and the duration and storage technique. From the clinical view, [PERSON] et al. [19] concluded that the addition of ZrO\({}_{2}\), TiO\({}_{2}\), and SiO\({}_{2}\) to light-cured RBCs could improve their mechanical and surface properties, but one of the most common issues seen regarding the reinforcement of light-cured RBCs via NPs is the color instability. Furthermore, this study added that modification of light-cured RBCs with ZrO\({}_{2}\), TiO\({}_{2}\), or SiO\({}_{2}\) can reduce their stainability, which is beneficial in prolonging restorative materials' lifetime and improving patient acceptance.