def ColorLuminance(color):
    """Compute the brightness of an sRGB color using the formula from
  http://www.w3.org/TR/2000/WD-AERT-20000426#color-contrast.

  Args:
    color: a string of 6 hex digits in the format verified by IsValidHexColor().

  Returns:
    A floating-point number between 0.0 (black) and 255.0 (white). """
    r = int(color[0:2], 16)
    g = int(color[2:4], 16)
    b = int(color[4:6], 16)
    return (299 * r + 587 * g + 114 * b) / 1000.0