It is impossible to give a full explanation of color "seasons" here so please read the article Color Seasons.
Think of falling leaves when you think of autumn. All the pure colors of summer have begun to turn brown or orange but all the colors are not pure brown. The colors are somewhere between the vivid summer and the soft earth. Another way you can think of autumn is to think earth tones. The majority of the autumn colors are earth tone shades of brown and orange. There are shades of blue, green and purple that are autumn colors but these are blues, greens and purples that are earth tones and mixed with a small amount of brown. Autumn colors are the most muted (most mixed) and the warmest (most golden) colors. They are often dark as well.