As mentionned by others, PIL is the right library. Here is a function that open an image and look for the main color.

def get_main_color(file):
    img = Image.open(file)
    colors = img.getcolors(256) #put a higher value if there are many colors in your image
    max_occurence, most_present = 0, 0
    try:
        for c in colors:
            if c[0] > max_occurence:
                (max_occurence, most_present) = c
        return most_present
    except TypeError:
        raise Exception("Too many colors in the image")

I hope it helps

Update : passing 256 to getcolors is ok for very small images but may not work in most cases. This value must be increased for bigger images. for example, 1024*1024 is ok for 400 pixels * 300 pixel image.