code: def background_color(self, node, depth):
          
          if self.color_mapping is None:
              self.color_mapping = {}
          color = self.color_mapping.get(node.key)
          if color is None:
              depth = len(self.color_mapping)
              red = (depth * 10) % 255
              green = 200 - ((depth * 5) % 200)
              blue = (depth * 25) % 200
              self.color_mapping[node.key] = color = wx.Colour(red, green, blue)
          return color
docs: Create a (unique-ish) background color for each node