From Fall Colors and Woodland Harvests - Laurel Hill Press

Leaves of some trees such as birches, tulip poplars, redbud and hickory, are always yellow in the fall, never red. The fall leaves of a few trees, including sugar maple, dogwood, sweet gum, black gum and sourwood, are usually red but may also be yellow.

Unlike the bright colors of flowers, which attract pollinators, or the bright "Warning colors" of many kinds of animals, the bright colors of fall foliage are a by-product of chemical changes as the trees start to go dormant. These colors have no apparent biological function or significance.

The most intense of fall color occurs in areas such as New England, with almost pure stands of a few types of trees, such as maples and birches, that all turn color at the same time during the short fall season.

The most varied fall color, as well as the longest lasting, occurs in areas such as the southern Appalachians, where a dozen or more kinds of trees may change color at slightly different times over the longer fall season.

The change in day length (photoperiod) that causes the chemical changes in the trees leading to the bright colors starts June 21, the longest day of the year, as the sun starts to move south and the days become shorter.

Leaves have just as much yellow pigment (xanthophyll) in July when they are green as they do in October when they are yellow. In July the darker green pigment (chlorophyll) masks the yellow color.

Evergreen trees may shed their older leaves, which often turn bright yellow, in spring rather than fall, but they never drop all their leaves at one time, thus staying green all year.

The leathery evergreen leaves of rhododendron are shed individually from time to time over several years; it is not uncommon to find individual rhododendron leaves that have been on a plant for five or six years that are still green, healthy and functional.

Bright sunlight is essential for the production of the red (anthocyanin) pigment in the fall leaves: if a black mark is placed on part of a leaf before it turns red, the part of the leaf under the mask will turn yellow while the exposed part will turn red.