I always get a little awe struck when I watch the Discovery channel and they explain that that seemingly mundane phenomena that I observe every single day is actually a relatively complex physics process. Like the sun bursting with flares or the moon rotating around the planet. There are so many intricately weaved cycles relying on something we so take for granted.
Well, leaves are one of those things. To me, with my artistically minded right brain in charge, I tend to focus on the fact that the whole world looks like a painting come to life. I'm feeling more than thinking when I take a stroll in the autumn leaves. But I imagine a lot of people look at the leaves and see the science. Like "Boy, look at all the glucose in that tree" or something like that.
For those of us interested in the science part I'm going to run down the list and explain, as it's been explained to me, why the leaves change color.
Green: The reason we see green everywhere we look for the duration of the year is because those tress- those amazing oxygen-producing factories that they are- are busy doing photosynthesis. Photosynthesis is the process by which trees turn carbon dioxide, sunlight and water into oxygen (for us) and glucose (for the trees). Chlorophyll is the tree's natural fuel for this process and it's what gives the leaves their green color.
Red and purple: When the season picks up and the days get shorter the trees gear up for hibernation. So, they stop doing photosynthesis and start saving up their energy for the long winter. As the chlorophyll fades from the leaves all the other chemicals change their pigments. That bright red and deep purple colors we see comes from Anthocyanins which are chemicals that also lend color to cranberries, blueberries and the like. These colors are most commonly seen in oak,dogwood and sugar-maple trees.
Yellow, Orange and Gold: These colors come from Carotenoids which also come in carrots and bananans. They're most commonly seen in aspen, yellow-poplar and black maple trees.
All the different trees have different timing to begin their hibernation so you get a steady stream of color across the few (far too few in my opinion) months of autumn. In general, the more moisture the trees have gotten and the more mild the temperature change (i.e. the temperature gets cool rather than freezing) the more glucose gets trapped in the leaves. The more glucose in the leaves, the more brilliant the color.
There are, of course, a lot more variables to consider when looking at color- soil composition, local vegetation, amount of air pollution, etc. But you can watch the discovery for that.