code: def merge_mean_color(graph, src, dst):
      """Callback called before merging two nodes of a mean color distance graph.
  
      This method computes the mean color of `dst`.
  
      Parameters
      ----------
      graph : RAG
          The graph under consideration.
      src, dst : int
          The vertices in `graph` to be merged.
      """
      graph.nodes[dst]["total color"] += graph.nodes[src]["total color"]
      graph.nodes[dst]["pixel count"] += graph.nodes[src]["pixel count"]
      graph.nodes[dst]["mean color"] = (
          graph.nodes[dst]["total color"]
          / graph.nodes[dst]["pixel count"]
      )
  
imports: from skimage import data, io, segmentation, color
  from skimage import graph
  import numpy
label_desc: Use image node_id 2 for calling a global function with example usage: merge_mean_color(graph, src, dst) without return types