from PIL import PngImagePlugin, Image

from collections import defaultdict

def get_dominate_color(image: PngImagePlugin.PngImageFile):

    colors = defaultdict(int)

    for pixel in image.getdata():

        colors[pixel] += 1

    return list(colors)[0]

>>> from PIL import Image

>>> im = Image.open('polar-bear-cub.jpg')

>>> from collections import defaultdict

>>> by_color = defaultdict(int)

>>> for pixel in im.getdata():

...     by_color[pixel] += 1

>>> by_color

defaultdict(<type 'int'>, {(11, 24, 41): 8, (53, 52, 58): 8, (142, 147, 117): 1, (121, 111, 119): 1, (234, 228, 216): 4