Roast labels sound precise, but light, medium and dark do not have universal definitions. Roasters use different machines, measurements and house styles. It is more useful to understand the direction in which flavour and brewing behaviour tend to change.
As coffee roasts longer and develops further, heat transforms its acids, sugars and aromatic compounds. Lighter roasting tends to preserve more of the differences associated with origin, variety and processing. These coffees may show brighter acidity, floral aromas or distinct fruit character.
Darker roasting shifts the balance toward flavours created during roasting. Chocolate, caramelised, nutty, smoky or bitter notes can become more prominent. The coffee also becomes more soluble, which means water can extract material from it more readily.
Medium roast covers a particularly wide range. It often aims to balance origin character with sweetness and roast development, but the label alone says little about where that balance sits.
Colour is one clue, not a complete measurement. The outside and inside of a bean can develop differently, and surface oil is affected by roast development, bean structure and time since roasting. A photograph is therefore not enough to classify every roast reliably.
Roast level also changes how a coffee responds to brewing. A dense, lightly roasted coffee may need finer grinding, hotter water or more contact time than a more developed coffee. A darker roast may become harsh or bitter under the same conditions. These are adjustment directions, not fixed recipes.
When comparing coffees, use the roaster's label together with what you taste. Record whether the cup feels bright, sweet, roasty, bitter, delicate or heavy. Over time, your own notes will make roast labels much more meaningful.