Image quantize - http://pickbox.ru/e/330

example.py

import quantize

image.open('test.png')

# Quantize image with 8 colors (3-bit)
image = quantize.quantize(image, quantize.PALETTE_8_COLORS)e

image.save('test_quantize.png')

quantize.py

from PIL import Image

PALETTE_8_COLORS = (
    (0, 0, 0), (0, 0, 255), (0, 255, 0), (0, 255, 255), (255, 0, 0), (255, 0, 255), (255, 255, 0), (255, 255, 255))


def quantize(im, use_colors=None):
    """
    Quantize image

    :param image:      Image for quantize
    :param use_colors: The list of colors, all other colors are converted into their
                       Each color must be presented as a tuple (red, green, blue)
    """

    # if use_colors is None and depth is None:
    #     return im

    palette = []

    # Generate palette from use_colors params
    if use_colors is not None:
        for color in use_colors:
            palette += color

    # The palette sequence must contain 768 integer values
    palette += [0, ] * (768 - len(palette))

    # a palette image to use for quantize
    palette_image = Image.new("P", (1, 1), 0)
    palette_image.putpalette(palette)

    # quantize it using our palette image
    return im.quantize(palette=palette_image)