Why are there so few red-leaved trees at this point? Does it have to do with the elongated warm weather?
Chlorophyll, which gives leaves their green color, breaks down in the fall when temperatures drop and days get shorter. Yellow and orange pigments (carotenoids) that have always been present in the leaves now become visible as the green fades. You will notice that yellow pigments tend to stay the same year after year, while reds and purples vary in their intensity and depend on seasonal factors.
Red and violet pigments are formed when sugars become trapped in the leaves. During sunny days sugars are produced in the leaves. In the cool fall nights, the veins in the leaves close up and prevent the sugars from moving into the tree. This excess of sugar encourages the production of anthocyanins.
Red pigments (anthocyanins) develop only in the fall. Rainfall, wind, temperatures and the amount of sugar in the leaves all influence the intensity of the foliage color. Rainfall during the growing season creates large leaves. Cool, bright, sunny days and chilly nights of fall (with no frost) create the brightest colors.
Your guess is correct. The warm weather has extended well into a period of much shorter daylight so trees have already been triggered to reduce photosythesis and get ready for dormancy. Fewer sugars are building in the leaves as the night temperatures drop, creating less brilliant red color. We also went quickly from warm weather to night freezes this year which limited the time for color to develop.
Courtesy of NYBG Plant Information