## 2 Materials and Methods
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To ensure complete polymerization, each specimen was light cured with a light-curing machine (Mega Physio Dental, Rastatt, Germany) for 20 sec on each side, and the curing probe was placed vertically to the specimens' surface at zero distance. Furthermore, Mylar strips were detached, and finishing burs (SofLex, 3M ESPE, Dental Products, Saint Paul, MN, USA) were used to remove any excess. For the purpose of surface standardization, each specimen was polished with grit-600 and grit-800 silicon sandpaper (3M ESPE, Saint Paul, MN, USA) attached to a polishing machine (MetaServ 250 Grinder-Polisher, Buehler, Lake Bluff, IL, USA) to standardize the amount and direction of pressure applied during polishing and to avoid excessive heat generation, which may lead to specimen distortion, as described in the previous study [9]. Subsequently, a visual examination of all specimens was performed, and any specimen with surface defects, warpage, porosities, or broken edges was excluded from the study. The rest of the samples were stored in distilled water for 48 h at 37 \({}^{\circ}\)C before starting the experimental tests [3; 35]. Before immersion, color measurements (L*, a*, and b* values) were performed for all specimens using a calibrated reflective spectrophotometer (X-Rite, model RM200 QC, Neu-Isenburg, Germany) at baseline, recording color parameters L1, a1, and b1 following the methods detailed previously [27; 36]. Regarding immersion solutions, Table 2 summarizes each solution preparation and immersion time/temperature. The pH of each freshly made immersion solution was measured by a pH meter (Orion Dual Star, pH/ISE meter, Thermo Scientific Inc., Waltham, MA, USA) (Table 2). Subsequently, each specimen was attached to a thread and then placed into the assigned jar that was filled with the immersion solution. According to the manufacturer of the coffee, the average consumption time for one cup of coffee is 15 min, and among coffee drinkers, the average consumption quantity is 3.2 cups per day. Therefore, a 24-h storage time simulates about 1 month of coffee drinking. Specimens were immersed for 6 days, simulating beverage consumption of 6 months [4; 37], and the jars were kept in an incubator at 37 \({}^{\circ}\)C for the whole duration. The immersion solutions were prepared and replaced daily by an investigator to reduce variances/errors in methodology [4; 38]. After immersion, all specimens were removed, gently rinsed with running distilled water for 5 min, and then left to dry. Then, the second color measurements were performed as formerly mentioned (L2, a2, and b2). The difference in color (\(\Delta\)E) was calculated based on the difference between recorded coordinate parameters before and after immersion using the following formula [39]:  
\[\Delta\text{E}^{*}_{\text{ab}}\ =\ \sqrt{\left(\text{L}_{2}^{*}\ -\ \text{L}_{1}^{*} \right)^{2}\ +\ \left(\text{a}_{2}^{*}\ -\ \text{a}_{1}^{*}\right)^{2}\ +\ \left(\text{b}_{2}^{*}\ -\ \text{b}_{1}^{*}\right)^{2}}\] \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Beverage** & **Company** & **Preparation of Beverage Solution** & **S soaking Time** & **Temperature** & **pH** \\ \hline Coffee & Abu Auf, New Cairo, Egypt & 200 mL distilled water boiled for 2 min then used to made 2 g of coffee, the mix then was filtered before usage. & & & 5.0 \\ \hline Tea & Lipton, London, UK & 200 mL distilled water boiled for 2 min then used to made 2 g of tea, the mix then was filtered before usage. & & & 6.1 \\ \hline Cola-Cola & Coca-Cola Co., Atlanta, GA, USA & & Ready made & & & 2.7 \\ \hline Water & Dasani, Atlanta, GA, USA & & & & & 7.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Protocol of beverage preparation.