Roast color determines 80% of a coffee's flavor.
Like a piece of bread, raw coffee is mostly carbohydrates like starches and oils, although there are over 1600 chemical compounds in a single coffee bean. When a slice of bread is toasted, it browns until it burns. Toasted light, you can taste the wheat. Toasted until burnt or very dark, you taste only the charred remains. So it is with our coffee bean (seed). The flavor changes with the degree of roast. A light roast Colombian tastes more like a light roast Nicaraguan than like a medium roast Colombian coffee.
Roast color is a function of temperature and time. Relatively speaking, artisan roasters use variations of these controllable factors to create flavor. Roasting is a craft much like pottery is. Two potters using the same clay, the same glazes, and the same shapes will have different outcomes. Temperature + time in the kiln will determine what the craftsman’s effort will produce. So it is with coffee. Generally, the coffee begins to roast at 405°F when the starches can get no hotter and they break down into simple sugars that carmelize at about 420°F. That is when the light roast is pulled or dumped. It can take from 8-14 minutes to get to a light roast color.
Between 420° and 475°F the color darkens until nothing is left to taste except burned plant matter!