you can set the SelectedMaterial property of the StLModelVisual3D to a new Material. Your code will look something like this -
StLModelVisual3D selectedModel = myHelixViewport3D.Children.OfType<StLModelVisual3D>().FirstOrDefault(m => m.IsSelected);
DiffuseMaterial selectedMaterial = new DiffuseMaterial(new SolidColorBrush(Colors.Red));
selectedModel.SelectedMaterial = selectedMaterial;