Autumn does not paint a leaf. It removes the green that was covering the yellow — while the reds are built fresh, on purpose. Here is the whole sequence.
Chlorophyll absorbs red and blue light and reflects green. A leaf makes it non-stop all summer, and while it does, that green drowns out every other colour in the leaf.
The trigger is night length, not cold. Once nights pass a certain threshold, the tree stops replacing chlorophyll — the same signal each year, whatever the weather does.
At the base of every leaf stalk, a layer of cells thickens into a seal. It chokes off the water going in and the sugar coming out, and it will eventually cut the leaf loose.
Chlorophyll is fragile and breaks down within days once it stops being replaced. It goes unevenly, which is why one leaf can still be green at the veins and yellow at the edges.
Carotenoids sat in the leaf all along, helping catch light. They outlast chlorophyll, so when the green goes, the yellow and orange underneath simply become visible.
Red is different: it did not exist in the summer leaf. Sugar trapped behind the seal is converted into anthocyanins in autumn — a pigment the tree spends energy to build.
Bright days make plenty of sugar and cool nights above freezing keep it trapped in the leaf. That pairing gives the deepest reds. Drought or an early hard frost dulls them.
Maples and oaks build anthocyanins and turn red. Birches, aspens and hickories mostly do not, so they run yellow. It is genetics — which is why one hillside shows both at once.
Before letting go, the tree pulls nitrogen and phosphorus back out of the leaf and stores them for spring. Only then does the seal finish its cut, and the leaf falls.
Northern Vermont and the high Rockies can peak from late September, and most of the US peaks through October. Elevation beats latitude — go up a mountain to see it early.