## 4 Discussion
However, in the same interval, the mean \(\Delta\)E\({}_{00}\) values of other tested material groups were observed below the perceptibility threshold value of 1.30. While VM had the significantly lowest mean \(\Delta\)E\({}_{00}\) values, followed by FP. The mean \(\Delta\)E\({}_{00}\) value of VE was statistically similar to the FP and CS, and greater than the VM after thermocycling at T1. The color stability of resin-based composites is affected by the hydrophobic or hydrophilic features of the resin matrix. This feature determines the water absorption capacity of the composite material [61]. Vita Mark II, a feldspathic ceramic, consists of a glass-ceramic matrix that is more resistant to discoloration than resin composites, as this matrix is not subject to water absorption like polymers [62]. The lowest color change of the VM after thermocycling can be attributed to its microstructure. The color change of the VE was statistically similar to that of the FP and CS and greater than VM at T1. Although VE is composed of a feldspathic glass ceramic of 86%, urethane dimethacrylate (UDMA) and tri-ethylene-glycol-dimethacrylate (TEGDMA)-containing polymer matrix of 14% in its structure may be the reason for its lower color stability compared to VM. Moreover, TEGDMA is hydrophilic and TEGDMA-based composites are more susceptible to color change due to their high water absorption capacity [63]. The organic matrix of CS contains UDMA and Bis-MEEP. There is a Bis-EMA matrix monomer in the structure of FP and SC. These monomers have lower hydrophilic properties compared to TEGDMA [22; 64]. In the present study, the hydrophobic monomer content of the CS and FP composite resins may be the reason for the lower color change. The interesting finding was that SC showed a significantly higher mean \(\Delta\)E\({}_{00}\) value than FP, although both FP and SC were additively manufactured composite resins with similar compositions according to manufacturers' statements (Table 1) at T1. Similarly, SC had a higher mean \(\Delta\)E\({}_{00}\) value than FP and other materials tested after immersion in coffee. Unlike the present study, [PERSON] et al. [36] found that the color change of Saremco Crowntec was similar to that of milled composite (CS) and was lower than that of other 3D printed resins tested (Varseo Smile) after 10,000 coffee thermocyclings. However, they found that the mean \(\Delta\)E\({}_{00}\) values of both 3D printed materials were within the clinically acceptable threshold. Whereas, [PERSON] et al. [22] reported that the color change values of another 3D printed composite resin were quite high compared to milled composite material after immersion in coffee solution. In the present study, 3D print group specimens were produced with different techniques. There is no published study comparing the effect of aging and storage in different beverages on the color stability of 3D printed permanent composite resins produced with DLP and SLA techniques. A previous study on the color stability of 3D printed interim resin materials produced with different techniques reported that the production technology of 3D printed materials may affect color stability. According to the results of that study, the 3D printing SLA group showed higher color stability than the DLP group [23]. Similarly, the differences in the production technology of 3D printing materials may have affected the color stability of the printing materials tested in the present study. In addition, the unreacted residual monomer is another factor that will cause color change even before the material is exposed to colorants. The post-polymerization process, which is a stage in the production of 3D printed materials, improves the material's properties by polymerizing unreacted monomers [65; 66]. In the present study, the different equipment used for the post-curing process and the different duration of the post-curing for each type of additively manufactured material may be the cause of the results [67; 68].