if Node.Index mod 2 = 0 then
begin
  TargetCanvas.Brush.Color := clBlack;
  TargetCanvas.FillRect(CellRect);
end
else
begin
  TargetCanvas.Brush.Color := clPurple;
  TargetCanvas.FillRect(CellRect);
end;

function GetVisibleIndex(Tree: TBaseVirtualTree; Node: PVirtualNode): Integer;
var
  P: PVirtualNode;
begin
  Assert(Assigned(Node), 'Node must not be nil!');
  Assert(Tree.IsVisible[Node], 'Node must be visible!');

  Result := 0;

  P := Tree.GetFirstVisible;
  while Assigned(P) and (P <> Node) do
  begin
    Inc(Result);
    P := Tree.GetNextVisible(P);
  end;
end;