As the bean absorbs heat, the colour shifts from green to yellow and then to varying shades of brown.
During the later stages of roasting, oils appear on the surface of the bean, making it shiny. The roast will continue to darken until it is removed from the heat source.
At lighter roasts, the bean will exhibit more of its "origin flavour" - the flavours created in the bean by the soil and weather conditions in the location where it was grown.
As the beans darken to a deep brown, the origin flavours of the bean are eclipsed by the flavours created by the roasting process itself.
At darker roast levels, the "roast flavour" is so dominant that it can be difficult to distinguish the origin of the beans used in the roast.
A note on flavour:
Describing the tastes of different roasts is very subjective.
In all cases there’s no substitute for your own personal taste. As a guide, if you can see the oil on the beans, you are more likely to taste the roasting flavours than the individual characteristics of the beans.
Cinnamon roast, half city, New England
After several minutes the beans “pop” or "crack" and visibly expand in size. This stage is called first crack. American mass-market roasters typically stop here.
Lighter-bodied, higher acidity, no obvious roast flavour
Full city, American, regular, breakfast, brown
After a few short minutes the beans reach this roast, which U.S. specialty sellers tend to prefer.
Sweeter than light roast; more body exhibiting more balance in acid, aroma, and complexity.
High, Viennese, Italian Espresso, Continental
After a few more minutes the beans begin popping again, and oils rise to the surface. This is called second crack.
Somewhat spicy; complexity is traded for heavier body/mouth-feel. Aromas and flavours of roast become clearly evident.
After a few more minutes or so the beans begin to smoke. The bean sugars begin to carbonise.
Smoky-sweet; light bodied, but quite intense. None of the inherent flavours of the bean are recognisable.
Grades of coffee roasting; from left: Un-roasted (or "green"), light, cinnamon, medium, high, city, full city, Italian, and French.