## 2 Materials and Methods
### Preparation of the Sample
The crowns were kept wet to optimize the color reading and the light reflection. One liter of laboratory-prepared artificial saliva solution containing water, KH2 PO4, KCl, NaCl, and Tris Buffer with pH = 7.0 was used in the control group [29]. The coffee solution (Al Khair Turkish Original Taste Coffee, brown) was prepared according to the manufacturer's instructions. One cup of water was put in a small coffee pot, and two small spoons of the coffee were added and stirred over a low heat. To record the baseline color coordinates (L0, a0, and b0), the images of all crowns in the 3 groups E-max (Ivoclare Vivadent, Vienna, Austria), Lisi (GC corporation, Tokyo, Japan), or Celtra (Dentsply Sirona Inc., Fort Worth, TX, USA) were captured with the Spectro Shade Micro and stored in the device's SD card. After that, each group was subdivided into 2 subgroups. The samples in the control subgroups were immersed in artificial saliva at 37 \({}^{\circ}\)C in incubator (HWB Series, Electric Thermostatic Water Bath, West Tune, Hangzhou, China) for 54 h, equivalent to 3 years of clinical simulation [18]. Meanwhile, the samples in the experimental subgroup were immersed in the prepared coffee solutions in an incubator for 54 h. The samples were immersed individually in 24-well cell culture plates inside the incubator [18]. For color measurement after immersion, the crowns were removed from the coffee and saliva and rinsed with distilled water to remove any residue. Photos were then taken with the device. Using the compare tool integrated into the Spectro Shade device (Figure 3), the baseline crown photos were compared with the crown photos after staining. Baseline color coordinates (L0, a0, b0), indicated on the left, and the color coordinates after staining (L1, a1, b1), indicated on the right, were calculated automatically and displayed on the device screen. In addition, the color difference (\(\Delta\)L1, \(\Delta\)a1, \(\Delta\)b1) and the overall color change, \(\Delta\)E1, was automatically calculated and displayed. It was considered that \(\Delta\)L1 = L1 \(-\) L0, \(\Delta\)a1 = a1 \(-\) a0 and \(\Delta\)b1 = b1 \(-\) b0. An outstanding feature of this tool is that the shade of an automatically generated equal area of the two crown photos can be compared, as shown in Figure 3. For repeatability, we selected a square area of the crown labial surface where the upper side of the square was formed by a line connecting the mesial and distal tips of the interdental papillae, and the lower side was formed by the line representing the incisal edge of the crown. To measure color change after brushing, the crowns were brushed with toothpaste and artificial saliva using an electric toothbrush (Oral-B Professional Care OxyJet +3000; Braun, Frankfurt, Germany). The standardized speed and pressure of the brushing were 59.5 rpm and 2 N, respectively [30]. One operator performed the brushing for all specimens. Again, the crown photos after brushing were compared with the photos after staining using the same procedure, in which the following parameters were automatically calculated and displayed on the device screen: L2, a2, b2, \(\Delta\)L2, \(\Delta\)a2, \(\Delta\)b2, and \(\Delta\)E2. It was considered that \(\Delta\)L2 = L2 \(-\) L1, \(\Delta\)a2 = a2 \(-\) a1 and \(\Delta\)b2 = b2 \(-\) b1. IBM SPSS software package version 20 (IBM Corp., Armonk, NY, USA) was used for statistical analysis. The Kolmogorov-Smirnov test was used to test for normality. A repeated-measure ANOVA with [PERSON]'s post hoc test was conducted to analyze the color coordinates L, a, and b. Meanwhile, a one-way ANOVA was employed to examine the color change variables (\(\Delta\)E, \(\Delta\)L, \(\Delta\)a, \(\Delta\)b), followed by [PERSON]'s post hoc test to determine significant differences between the groups. The significance level was set at \(p\leq 0.05\).