Used this script to minimalize osm-bright mml colors to grayscale for creating a data base layer map in tilemill.

hexcolor2grayscale.py

"""
Script to replace color #RRGGBB formated values to grayscale values inplace.

usage:
    python hexcolor2grayscale.py filename1 filename2
"""
import fileinput
import sys
import re
import colorsys

def color2grayscale(values):
    rgb = None
    if values:
        # process as hex (ignore alpha)
        # values 0-255
        if len(values) == 3:
            # (3 digit) patterns are converted by doubling each hex digit
            # --> #abc is equivalent to #aabbcc
            rgb = [int(v+v, 16)/255 for v in values]
        elif len(values) == 6:
            rgb = [int(values[i:i+2], 16)/255 for i in range(0, 6, 2)]
    result = values
    if rgb:
        luma, i, q = colorsys.rgb_to_yiq(*rgb)
        # get just the luma channel, convert back to rgb
        gray_rgb = colorsys.yiq_to_rgb(luma, 0, 0)
        result = "#{}".format("".join("{:02X}".format(int(v * 255)) for v in gray_rgb))
    return result

_hex_colour = re.compile(r'#([0-9a-fA-F]{3}|[0-9a-fA-F]{6})\b')

def replace(match):
    value = match.group(1)
    if value:
        result = color2grayscale(value)
    else:
        result = value
    return result

for line in fileinput.input(inplace=True):
    line = _hex_colour.sub(replace, line)
    sys.stdout.write(line)