## 4 Discussion  
This study evaluated the effect of four staining solutions (red wine, black tea, coffee, and Coke(r)) on the color stability of three denture base dental polymers (heat-polymerizing PMMA resin, hard-type 3D-printed PMMA resin, and urethane-based soft 3D-printed resin) in three immersion periods (one day, one week, and one month). The null hypothesis of this study was rejected by the results in most comparisons, indicating the significant influence of all parameters applied on the \(\Delta\)E of all materials. There is a direct and strong influence between immersion time and color change in the tested materials. This agrees with many studies that confirm the strong relationship between exposure time and coloring effect [18; 22; 29; 30; 31; 32; 33; 34; 35; 36]. There was a slight increase of \(\Delta\)E at the end of the 7-day period, which increased further at the end of the 30-day period (Figure 4, Figure 7, and Figure 8), but the values were at the 1.0 Unit level after immersing in water, coffee, and Coke(r). The black tea and red wine solutions showed a stronger influence during all periods at the level of approximately 4.0 Units for black tea and 2.9 Units for the red wine at the end of the 30-day period. The same behavior was apparent in all time periods. Many studies mention that water absorption is a common possible mechanism for discoloring denture base resins [19; 31; 32; 37]. [PERSON] et al. [37] found that 3D-printed resins absorbed water molecules, leading to an increase of surface solubility and roughness. The chemical bonds of the polymer network are decomposed from the hydrolysis effect of water, and a surface precipitation of coloring pigments becomes possible. So, the presence of water is critical over time and acts as a possible leader or a contributing factor to enhance various colorant's deposition [31; 38]. On the other side, some studies [19; 39] found a decrease of \(\Delta\)E values after a 7-day immersion period. They propose that, after a period, there is a stabilizing mechanism of PMMA resins through an aging process. Therefore, this point needs to be confirmed by clinical studies. [PERSON] et al. [29] mention that the discoloration process happens to a larger extent in the initial phases of exposure in stains, with coffee being the strongest discoloring media. Concerning the different solutions, the results of this study showed an aggressive discoloring effect of black tea and red wine solutions on all materials. All materials in all immersion periods had an increasing \(\Delta\)E, with the highest values shown in the 30-day period. In pairwise comparisons, there were significant (\(p<0.05\)) differences for black tea and red wine compared with the other solutions and with each other. Otherwise, black tea was the strongest discoloring media, followed by the red wine solution. Black tea contains color pigments with small (catechins, theaflavins) and large molecular weights (tannins, thearubigins). They also have an acidic nature and high-water solubility [18]. Therefore, under the presence of the water environment, which contributes to resin degradation and absorption, these colorants have a critical influence on the materials under this study's conditions. This study corroborates the results of [PERSON] et al. [31], that tea discolors denture base resins over time, with lower values than red wine and Coke(r). [PERSON] et al. [17] also found that tea affects the color stability of 3D-printed resin over time. Red wine is a well-established discoloring media in the oral environment. Many studies confirm this finding in comparison with various media (Coke(r), coffee) [30; 31; 32; 33; 34; 30]. Red wine has an acidic influence because it contains alcohol acids (pH-3.9), so it roughens the resin's surface [33]. The alcohol molecules are absorbed by the organic matrix and have a plasticizing effect on the resin surface. The color pigments, like tannins, minerals, and anthocyanin, which is water soluble, can be more easily deposited.