This is by design. When a Series element's color is controlled by the Series.Color property, then neither that Series's Brush nor Pen can alter it. So to change the color of the Vector3D series:

    private void button4_Click(object sender, EventArgs e)
    {
      series.UseColorRange = false;
      series.Color = Color.Red;
    }

You can see this logic working in the Line series, e.g.

    Line series;
    private void InitializeChart()
    {
      tChart1.Aspect.View3D = false;
      series = new Line(tChart1.Chart);
      series.FillSampleValues();

    }

    private void button4_Click(object sender, EventArgs e)
    {
      series.LinePen.Color = Color.Red;
    }