Maple syrup is graded by light transmittance, which roughly corresponds to flavor and when during the season it is produced.
100-75% light transmittance is graded as golden.
50-74.9% is amber.
25-49.9% is dark.
0-24.9% is very dark.
Each grade has a band of light transmittance that is 25% wide, so it is possible to have amber that is almost golden or on the frontier of dark. Medium and large producers can blend different grades to make syrup that is close to the middle of the color range. Artisanal sugarmakers often have each production day stand on its own because we fill bottles, not barrels. No blending means there is more variation in colors. But we can't taste color.
Ambient temperature and natural fermentation of the sap cause the color to change. The season begins when nights are freezing and the days start to warm up. Every day that there is a freeze-thaw cycle is like a pump for the sap. The warm days also encourage the bacteria and yeasts that naturally occur in the environment and sap to transform sugars and elements in the sap into compounds we can taste and smell.
The warmer the temperatures during the day, the more advanced the fermentation. One example - maple syrup contains diacetyl, which is the compound that makes butter taste like butter. It is also present in Chardonnays, baked goods, cheese, milk, yogurt, whiskey, and roasted coffee. The arc of diacetyl in maple syrup begins with the taste of fresh butter in golden when ambient temperatures are just above freezing then evolves into melted butter in amber and butterscotch in dark when daily temperatures are in the 50F to 60s (10-15 C).
Common flavors in amber are maple, caramel, melted butter, and vanilla. Less often, there can be notes of smoke, nutmeg, cinnamon, fruit, honey, and roses. Dark syrup has less complexity but is more full-bodied. Common flavors are intense maple, caramel, and butterscotch.