docs: def luminance(mycolour):
      r"""Determine (relative) luminance of a colour.
  
      Args:
          mycolour(colourettu.Colour): a colour
  
      Luminance is a measure of how 'bright' a colour is. Values are
      normalized so that the Luminance of White is 1 and the Luminance of
      Black is 0. That is to say:
  
      .. code:: pycon
  
          >>> colourettu.luminance("#FFF")    # white
          0.9999999999999999
          >>> colourettu.luminance("#000")    # black
          0.0
  
      ``luminance()`` can also be called on an already existing colour:
  
      .. code:: pycon
  
          >>> c3.luminance()
          0.2641668488934239
          >>> colourettu.luminance(c4)
          0.08007571268096524
  
      .. note::
  
          Uses the formula:
  
          \\[ lum = \\sqrt{0.299 r^2 + 0.587 g^2 + 0.114 b^2} \\]
      """
  
      colour_for_type = Colour()
      if type(mycolour) is type(colour_for_type):
          mycolour2 = mycolour
      else:
          try:
              mycolour2 = Colour(mycolour)
          except:
              raise TypeError("Must supply a colourettu.Colour")
  
      (r1, g1, b1) = mycolour2.normalized_rgb()
  
      return math.sqrt(0.299*math.pow(r1, 2) +
                       0.587*math.pow(g1, 2) +
                       0.114*math.pow(b1, 2))