Roasting gourmet coffee beans naturally changes both the physical and chemical characteristics of the coffee.
Barely roasting a green coffee bean to achieve a Light Roasted Coffee results in a more acidic coffee that has more caffeine and is lighter brown in color. Alternatively, roasting a green coffee bean for a long period of time results in a Dark Roast Coffee which is less acidic, has less caffeine and is a deep dark brown color with an oily appearance.
Medium roasted coffees fall somewhere in between these two and can have subtle caramel or cocoa flavors while maintaining a milk chocolate color