Plants make their own food. They take water from the ground through their roots. They take a gas called carbon dioxide from the air. They turn water and carbon dioxide into food (a kind of sugar) using sunlight for energy and something called chlorophyll. Chlorophyll is green, which is why leaves are green. 

In the fall, most plants in these latitudes stop making food.  Chlorophyl is no longer needed, so it is discarded.  That is when you see the other colors that the chlorophyl covered during the summer.  Red leaves are the result of food that remains in the leaves after the plant stops taking it in.  Brown comes from wastes not discarded before the whole process shut down.

Preserving fall leaves

There are several ways to save the beauty of fall in your home.  You can preserve a treasured leaf by placing it between two layers of wax paper, then covering it with an old towel or cloth rag.  Next, press the covering fabric with a warm iron.  This will seal the wax paper together with the leaf in between. Cut your leaves out, leaving a narrow margin of wax paper around the leaf edge and place them in a sill or on a table for display..

You also can preserve fall leaves in your microwave oven. This method works best with fresh leaves, still bright with color. A leave that has begun to dry will not turn out well. Take a leaf and put in in the microwave on top of two paper towels.  Cover it with a single paper towel.  It will take between a half a minute and two minutes on high, depending on how wet the leaf is.  If the leaf curls when you take it out, it hasn't been dried long enough.  Scorched leaves were cooked too long.  When you get it right, it can then be left to dry for a day or so, then coated with some kind of acrylic craft sealant.

(Another microwave method involves laying the leaves flat on top of a layer of floral silica gel in a small cardboard box, then apply another inch of the gel over them.  If you cook them at a medium-low setting (like defrost), the time for this method is two to five minutes)

Yet a third method bypasses cooking.  Submerge the leaves in a solution of one part glycerin to two parts water.  You'll have to put something on top to keep them submerged. In two to six days, they should have absorbed the liquid and be soft and pliable. Remove them from the pan and wipe off all the liquid with a soft cloth. They will stay soft and pliable indefinitely.