Unable to remove checkbox in front of parent node - treeView

I'm trying to hide the checkbox infront of all my parent nodes in my treeview, but I can't get it to work.  I also tried to modify the color of the text and background but that does not work either.  I tried placing the call to the function in the form load, also in the contructor. Here is the functions I am trying to call

//Hide Checkbox

private const int TVIF_STATE = 0x8;
        private const int TVIS_STATEIMAGEMASK = 0xF000;
        private const int TV_FIRST = 0x1100;
        private const int TVM_SETITEM = TV_FIRST + 63;

        [StructLayout(LayoutKind.Sequential, Pack = 8, CharSet = CharSet.Auto)]
        private struct TVITEM
        {
            public int mask;
            public IntPtr hItem;
            public int state;
            public int stateMask;
            [MarshalAs(UnmanagedType.LPTStr)]
            public string lpszText;
            public int cchTextMax;
            public int iImage;
            public int iSelectedImage;
            public int cChildren;
            public IntPtr lParam;
        }

        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        private static extern IntPtr SendMessage(IntPtr hWnd, int Msg, IntPtr wParam,
                                                 ref TVITEM lParam);

        /// <summary> 
        /// Hides the checkbox for the specified node on a TreeView control. 
        /// </summary> 
        private void HideCheckBox(TreeView tvw, TreeNode node)
        {
            TVITEM tvi = new TVITEM();
            tvi.hItem = node.Handle;
            tvi.mask = TVIF_STATE;
            tvi.stateMask = TVIS_STATEIMAGEMASK;
            tvi.state = 0;
            SendMessage(tvw.Handle, TVM_SETITEM, IntPtr.Zero, ref tvi);
        }

This is how I was colling it (in the constructor of my form)

foreach (TreeNode tn in treeView1.Nodes)
            {
                // if the text properties match, color the item 

                if (tn.Text == "NODE0")
                {
                    tn.BackColor = Color.Yellow;
                    HideCheckBox(this.treeView1, tn);
                }
            }