Установить свойство TreeView:
treeView1.DrawMode = TreeViewDrawMode.OwnerDrawText;
Затем из события DrawNode:
private void treeView1_DrawNode(object sender, DrawTreeNodeEventArgs e) {
Color nodeColor = Color.Red;
if ((e.State & TreeNodeStates.Selected) != 0)
nodeColor = SystemColors.HighlightText;
TextRenderer.DrawText(e.Graphics,
e.Node.Text,
e.Node.NodeFont,
e.Bounds,
nodeColor,
Color.Empty,
TextFormatFlags.VerticalCenter);
}
Больше от MSDN: TreeView.DrawNode Event