The reason that a leaf can be green is throughout spring and summer the green hue comes from a chemical chlorophyll, this color is vital to photosynthesis.
Along with green, there are yellow and orange colors, carotene and xanthophyll, inside some trees' leaves. For most of the year, these colors are are covered by chlorophyll, but as the chlorophyll breaks down and the green color dissipates, the yellow and orange colors become visible.
Another color` that occurs in leaves is the red or Anthocyanins. Anthocyanins, unlike carotene and xanthophyll, are not in leaves year-round. It isn't until the chlorophyll begins breaking down that the plant begins to synthesize anthocyanin. Very acidic sap results in a bright red color, while less acidic sap leads to a purplish red.
The humdrum or brown color is the result of waste products trapped in the leaves.