code: def _weight_mean_color(graph, src, dst, n):
      """Callback to handle merging nodes by recomputing mean color.
  
      The method expects that the mean color of `dst` is already computed.
  
      Parameters
      ----------
      graph : RAG
          The graph under consideration.
      src, dst : int
          The vertices in `graph` to be merged.
      n : int
          A neighbor of `src` or `dst` or both.
  
      Returns
      -------
      data : dict
          A dictionary with the `"weight"` attribute set as the absolute
          difference of the mean color between node `dst` and `n`.
      """
  
      diff = (
          graph.nodes[dst]["mean color"] - graph.nodes[n]["mean color"]
      )
      diff = np.linalg.norm(diff)
      return {"weight": diff}
  
imports: from skimage import data, io, segmentation, color
  from skimage import graph
  import numpy
label_desc: Use image node_id 1 for calling a global function with example usage: _weight_mean_color(graph, src, dst, n) and returns: dict