use mapnik.Image to dump out all rgba colors in an image

unique-colors.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import mapnik
import sys

def pixel2channels(pixel):
    alpha = (pixel >> 24) & 0xff
    red = pixel & 0xff
    green = (pixel >> 8) & 0xff
    blue = (pixel >> 16) & 0xff
    return red,green,blue,alpha

def pixel2rgba(pixel):
    return '%s,%s,%s,%s' % pixel2channels(pixel)

def get_unique_colors(im):
    pixels = []
    for x in range(im.width()):
        for y in range(im.height()):
            pixel = im.get_pixel(x,y)
            if pixel not in pixels:
                 pixels.append(pixel)
    pixels = sorted(pixels)
    return map(pixel2rgba,pixels)
    
if __name__ == "__main__":
    image = sys.argv[1]
    im = mapnik.Image.open(image);
    for idx,c in enumerate(get_unique_colors(im)):
        print idx,c