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).

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