To reproduce:
  for (int i = 0; i < 10; i++)
            {
                RadTreeNode node = new RadTreeNode("Node " + i);
                radTreeView1.Nodes.Add(node);

                node.Style.GradientStyle = GradientStyles.Solid;
                node.Style.BackColor = System.Drawing.Color.LightCoral;
            }
Workaround: use NodeFormatting