if(treeView1.SelectedNode.Nodes.Count > 1 && treeView1.SelectedNode.ForeColor != Color.Blue)
treeView1.SelectedNode.ForeColor = Color.Green;

//Add Siblings to treeview

TreeNode tvnode = new TreeNode("your_Node_Name");
tvnode.ForeColor = Color.Brown;
treeView1.SelectedNode.Parent.Nodes.Add(tvnode);

//Delete Selected Node
treeView1.SelectedNode.Remove();