Access different vertices of a Mesh Unity 3d

Read up on the Mesh class. You can use this to read & modify any information you want about mesh assets in your project, or even create whole new meshes procedurally.

You can access and iterate over mesh information like this:

void AssignBoneColors(Mesh mesh)
{
   Color32[] colors = mesh.colors32;
   // Ready an array large enough for all vertices if the mesh didn't already have one.
   if(colors.Length == 0)
       colors = new Color32[mesh.vertexCount];

   BoneWeight[] weights = mesh.boneWeights;

   // If the mesh has no skinning weights, this will be zero, 
   // which skips the loop below.
   int numVertices = weights.Length;

   for(int i = 0; i < numVertices; i++)
   {
       colors[i] = GetColorForBoneWeights(weights[i]);
   }

   mesh.colors32 = colors;
}

GetColorForBoneWeights() is where you'll implement your colour selection based on which limb the vertex is skinned to. Note that boneWeight.boneIndex0 should contain the index of the bone with the highest weight for each vertex, so you could colour based on that bone alone, or navigate to its first parent to which you've assigned a color, or write a colour-weighting function that considers multiple bones. These implementation details are up to you.

Note that the colours will only be visible if you render the mesh with a shader that uses vertex colours, or use additional editor extensions/gizmos to help visualize vertices.