Color Palette:
1. Color Name: Light Gray
   Hex Code: 
                            
                            #D3D3D3
                        
   Description: Represents "no usage" of tobacco. This color signifies a neutral and non-consumptive state.

2. Color Name: Sepia
   Hex Code: 
                            
                            #704214
                        
   Description: Represents "old usage" of tobacco. This warm brown color evokes a sense of nostalgia and historical context.

3. Color Name: Olive Green
   Hex Code: 
                            
                            #556B2F
                        
   Description: Represents "current consumption" of tobacco. This earthy green color conveys a sense of growth and vitality, while also hinting at the complexities and risks associated with current usage.

4. Color Name: Dark Gray
   Hex Code: 
                            
                            #A9A9A9
                        
   Description: Provides a neutral background or accent color for the scientific graph, creating a cohesive and professional visual presentation.

This color palette effectively represents the different stages of tobacco usage, providing a clear and visually appealing way to convey the information in a scientific graph.