Your comments

Is there a working way for transparency by now?

If you don't like the Labels having the same colors as the axis, remove the lines

zlabel.Foreground = new SolidColorBrush(this.ZAxisColor);

also maybe think about using zlabel.FontSize instead of zlabel.HeightFactor. I like the result more ;)

"x" is string

XAxisLabel has type Color... why?

XAxisLabelProperty is registered with type BillboardTextItem.. why?

Remember, in no way the coordinate system is geometry centered. It is just a coordinate system positioned at (0,0,0).

No, it's not implemented as far as I know.

What you could do is this:

<HelixToolkit:CoordinateSystemVisual3D />
<HelixToolkit:BillboardTextVisual3D Position="1.2 0 0" Text="X" /> <HelixToolkit:BillboardTextVisual3D Position="0 1.2 0" Text="Y" /> <HelixToolkit:BillboardTextVisual3D Position="0 0 1.2" Text="Z" />

Or simply write your own CoordincateSystemVisual-class. If you subclass the existing one it shouldn't be that hard.

Add a coordinate system urself, do not use the one of the helixviewport with center/center.

The one of the helixviewport is the global axis and always stays relative to the whole viewport.

Easiest way: in your xaml:

<!-- You can also add elements here in the xaml -->
<HelixToolkit:GridLinesVisual3D Width="8" Length="8" MinorDistance="1" MajorDistance="1" Thickness="0.01"/>

<HelixToolkit:CoordinateSystemVisual3D />

Maybe it's the same problem I had with the TruncatedConeVisual... Because no normals where generated, it was rendered black (most of the time, sometimes they had colors)

Try create your own class like so:

   public class MyEllipsoidVisual3D : EllipsoidVisual3D
       protected override MeshGeometry3D Tessellate()
           var builder = new MeshBuilder(true, true);
           builder.AddEllipsoid(this.Center, this.RadiusX, this.RadiusY, this.RadiusZ, this.ThetaDiv, this.PhiDiv);
           return builder.ToMesh();

For circles use short TubeVisuals3D

Black sounds like you don't have a light. Default color is blue