HelixToolkit: LinearGradient along a tube: How?

I try to show a tube with varying diameters and bends in it. I need to show, in color, the stresses in the tube. I would like to do that with a LinearGradientBrush, "painting" the tube along its length. Helix draws a nice tube. However I canot figure out "how to apply the paint". Here is some code I wrote :

        var path = new Point3DCollection() { new Point3D(0, 0, 0) };
        double[] diameters = new double[Members.Where(m => !m.Left).Count()+1];
        diameters[0] = Members.First().Diameter;
        int i = 1;
        foreach (var mb in Members.Where(m => !m.Left))
            diameters[i] = mb.Diameter;

        MeshGeometryVisual3D mgv = new MeshGeometryVisual3D();
        MeshBuilder mesh = new MeshBuilder();

        mesh.AddTube(path, null,diameters, Members.Where(m => !m.Left).Count(), false);           

        mgv.MeshGeometry = mesh.ToMesh();
        mgv.Fill = GetGradientBrush();

And here I generate the Brush:

public LinearGradientBrush GetGradientBrush()
   LinearGradientBrush Brush = new LinearGradientBrush() { StartPoint = new Point(0, 0) , EndPoint = new Point(1,1) };
   var Length = Members.Sum(m => m.Axis.Length);

   uint j;
   uint steps = (uint)Members.Count() + 1;
   var gs = new GradientStopCollection();
   var l = 0d;
   foreach (var mb in Members)
       gs.Add(new GradientStop(Member._spectrum[(int)(mb.StrengthA / MaxSStrength *
           Member._spectrum.Length) - 1], l / Length));
          l += mb.Axis.Length;

   gs.Add(new GradientStop(Member._spectrum[(int)(Members.Last().StrengthB / MaxStrength *
         Member._spectrum.Length)], 0));

    return new LinearGradientBrush(gs, 90);           

A point of interest is, that the color values may go up AND down, "crawling along the tube", so they are not only going EITHER up OR down like on a regular gradient. What happens is, that I get a tube in only ONE color, from one end of the spectrum (_spectrum is a long list of color codes, from which I pick). The form of the tube shown is good.

So: what am I doing/thinking wrong here? Is it at all possible, what I want?