Humidity and color-curing: During leaf senescence, the active destruction of chlorophyll (green) and the slower production of xanthocyanin (red) are both living processes. Carotinoids (yellow) already exist in the background, hidden by the chlorophyll. If the leaf remains alive long enough for the chlorophyll to break down, and is then rapidly dried, you get a yellow leaf. If the leaf is kept alive (not allowed to fully dry) for even longer, then xanthocyanins are produced more abundantly, and you get a deeper leaf color, from reddish yellow to deep maduro brown. So a prolonged, more humid color-curing condition result in darker leaf.
Practically speaking, most of us cannot exercise full control of color-curing conditions. So, knowing the finer details of the leaf physiology won't help us much. But understanding that the leaf must remain alive until it yellows is meaningful.
Bob