## 4 Discussion  
In the current study, the colour changes of 3D printed and heat-cured PMMA denture base resins were investigated. They were immersed in four staining agents for three periods of time--7, 14 and 21 days. The results demonstrated that the different duration affected the \(\Delta\)E of all tested specimens. Furthermore, the type of staining agent interacted extensively with the selected time periods. Therefore, the null hypothesis that there would be no major colour difference in any of the tested materials irrespective of the staining medium and duration of immersion can be rejected. Our survey results concur with the findings of other authors [44; 45; 46]. Many researchers studied dental materials placed in different beverages, which suggests the necessity for standardization of the method. In agreement with the study of [PERSON] et al., the colour stability of 3D printed denture resins was better compared to conventional PMMA for the selected periods of time [47]. In accordance with the study of [PERSON] et al., because a priori analysis provides a method of controlling type I and II errors for hypothesis testing, it is one of the best possible options for measuring sample size and power [48]. According to the research of [PERSON] et al., after one week, all of the specimens had significant colour changes, with a steady increase for the other immersion periods [49]. The discolouration effect of the artificial saliva was investigated by a number of authors, and the results showed that there was a slight decrease in the delta E values with the progress of time [50]. This rate shows only a light degree of discolouration, which indicates a negligible change in the optical stability of the tested specimens. [PERSON] et al. found that CAD/CAM fabricated resins for removable dentures are also responsive to hygroscopic dilation, which is caused by the two hydrophilic urethane units within their molecular compound, but less responsive than PMMA [51]. According to the survey of [PERSON] et al., coffee and coke were the staining solutions demonstrating the highest colouring effect without any major differences [52]. However, our findings showed the highest values for staining for the red wine for all immersion periods in both of the tested groups, which was in accordance with the studies of [PERSON] et al. and [PERSON] et al. [53; 54]. In contrast, in the study of [PERSON] et al., no difference was found in the colour stability among the two groups of tested experimental polished specimens immersed in various solutions [55]. This may be due to the different conditions and exposure times in the individual studies. According to the study of [PERSON] et al., the discolouration of different resins used in prosthetic dentistry after immersion in a variety of colouring solutions was higher than the clinically acceptable standard of \(\Delta\)E 3.3. This is in contrast with the current investigation results showing that the two groups of dental resins are clinically acceptable. The result of the research of [PERSON] et al. could be due to the predisposition of high water absorption in light-activated denture base resins when compared to the other materials [56]. In the present study, a higher colour difference for the two types of denture base resins was found for the red wine group compared to other observed solutions. The most probable cause is that red wine contains anthocyanin, which is a water-soluble pigment and provides the grapes with their colour [57]. According to [PERSON] et al., the other possible reason for staining by red wine is ethanol, due to the fact that it can plasticize the composite's surface [15]. In accordance with [PERSON] et al., water absorption is involved in the process of coffee colouring, while coke causes stains on the surface of the denture base only by adsorption [58]. Moreover, a variety of cleansing agents are capable of reducing the coloured areas to a great extent [59].