TreeNode node1 = new TreeNode("Node 1");
TreeNode node2 = new TreeNode("Node 2");
TreeNode node3 = new TreeNode("Node 3");
treeView1.Nodes.Add(node1);
treeView1.Nodes[0].BackColor = Color.Red;
treeView1.Nodes.Add(node2);
treeView1.Nodes[1].BackColor = Color.Green;
treeView1.Nodes.Add(node3);
treeView1.Nodes[2].BackColor = Color.Yellow;