## 2 Materials and Methods
### Specim Preparation (Color Stability)  
One hundred and sixty lower molar acrylic teeth of four different shades (B1, B2, A3, and C3) were prepared with standardized simulated class I preparations using a high-speed hand piece ([PERSON], Melville, NY, USA) and number 245 carbide burs ([PERSON], Melville, NY, USA). The preparation dimensions were 2 mm in depth and 4 mm in diameter. The prepared cavities were rinsed with water and air-dried. All tooth preparations were performed by a single operator (AB). Based on the four different teeth shades, the prepared teeth were divided into 4 groups of 40 teeth each (B1, B2, A3, and C3).  
Half of the prepared teeth (\(n=80\)) were restored with a single-shade OM (single-shade universal composite, Tokuyama Dental, Encinitas, CA, USA) (test group) and the other half (\(n=80\)) were restored with a conventional resin-based composite (Filtek Z-350 (FT), 3M technology, Minnesota, MN, USA) (control group). Based on the combinations of tooth shades, restorative materials, and staining mediums, the specimens were divided into 4 groups.  
Group 1. Filtek-Cola (\(n=40\)): Acrylic teeth restored with Filtek Z-350 resin-based composite were immersed in a carbonated drink (Pepsi, Pepsi Co., Harrison, NY, USA) for 2 weeks. This group consisted of four subgroups with four different acrylic tooth shades (B1, B2, A3, and C3). All teeth were restored with the corresponding color resin material (Filtek).  
Group 2. Filtek-Coffee (\(n=40\)): Acrylic teeth restored with Filtek Z-350 resin-based composite were immersed in coffee (Nestle USA, Inc., Glendale, CA, USA) for 2 weeks. This group consisted of four subgroups with four different acrylic tooth shades (B1, B2, A3, and C3). All teeth were restored with the corresponding color resin material (Filtek).  
Figure 2: Color matching assessment of restored tooth specimens from the different study groups.  
Group 3. OM-Cola (\(n=40\)): Acrylic teeth restored with OM resin-based composite were immersed in carbonated drink (Pepsi, Pepsi Co., Harrison, NY, USA) for 2 weeks. This group consisted of four subgroups with four different acrylic tooth shades (B1, B2, A3, and C3). All teeth were restored with the same shade of OM resin material.  
Group 4. OM-Coffee (\(n=40\)): Acrylic teeth restored with OM resin-based composite were immersed in coffee (Nestle USA, Inc., Glendale, CA, USA) for 2 weeks. This group consisted of four subgroups with four different acrylic tooth shades (B1, B2, A3, and C3). All teeth were restored with the same shade of OM resin material.  
Specimens in groups 1 and 3 were immersed in a carbonated drink, which was replaced every 24 h for 2 weeks [8]. To prepare the coffee solution, 25 grams of coffee (Nestle USA, Inc., Glendale, CA, USA) was poured into 250 milliliters of water at 100 \({}^{\circ}\)C. The solution was stirred for 10 min and filtered. The coffee solution was replaced every 24 h. Each specimen was assessed for baseline color after 24 h of restoration, prior to immersion in the stain solutions (carbonated drink and coffee). After removal from the stain solution, specimens were washed under running water for 5 min and dried. All specimens were assessed for color change after staining.