public class SegmentSymbolOptions 

//Custom tools have the ability to change the symbology used when sketching a new feature. 
//Both the Sketch Segment Symbol and the Vertex Symbol can be modified using the correct set method. 
//This is set in the activate method for the tool.
protected override Task OnToolActivateAsync(bool active)
{
  QueuedTask.Run(() =>
  {
    //Getting the current symbology options of the segment
    var segmentOptions = GetSketchSegmentSymbolOptions();
    //Modifying the primary and secondary color and the width of the segment symbology options
    var deepPurple = new CIMRGBColor();
    deepPurple.R = 75;
    deepPurple.G = 0;
    deepPurple.B = 110;
    segmentOptions.PrimaryColor = deepPurple;
    segmentOptions.Width = 4;
    segmentOptions.HasSecondaryColor = true;
    var pink = new CIMRGBColor();
    pink.R = 219;
    pink.G = 48;
    pink.B = 130;
    segmentOptions.SecondaryColor = pink;
    //Creating a new vertex symbol options instance with the values you want
    var vertexOptions = new VertexSymbolOptions(VertexSymbolType.RegularUnselected);
    var yellow = new CIMRGBColor();
    yellow.R = 255;
    yellow.G = 215;
    yellow.B = 0;
    var purple = new CIMRGBColor();
    purple.R = 148;
    purple.G = 0;
    purple.B = 211;
    vertexOptions.AngleRotation = 45;
    vertexOptions.Color = yellow;
    vertexOptions.MarkerType = VertexMarkerType.Star;
    vertexOptions.OutlineColor = purple;
    vertexOptions.OutlineWidth = 3;
    vertexOptions.Size = 5;

    //Setting the value of the segment symbol options
    SetSketchSegmentSymbolOptions(segmentOptions);
    //Setting the value of the vertex symbol options of the regular unselected vertices using the vertexOptions instance created above.
    SetSketchVertexSymbolOptions(VertexSymbolType.RegularUnselected, vertexOptions);
  });

  return base.OnToolActivateAsync(active);
}

//var options = ApplicationOptions.EditingOptions;
QueuedTask.Run(() =>
{
  var seg_options = options.GetSegmentSymbolOptions();
  //to convert the options to a symbol use
  //SymbolFactory. Note: this is approximate....sketch isn't using the
  //CIM directly for segments
  var layers = new List<CIMSymbolLayer>();
  var stroke0 = SymbolFactory.Instance.ConstructStroke(seg_options.PrimaryColor, 
    seg_options.Width, SimpleLineStyle.Dash);
  layers.Add(stroke0);
  if (seg_options.HasSecondaryColor) {
    var stroke1 = SymbolFactory.Instance.ConstructStroke(
      seg_options.SecondaryColor, seg_options.Width, SimpleLineStyle.Solid);
    layers.Add(stroke1);
  }
  //segment symbology only
  var sketch_line = new CIMLineSymbol() {
    SymbolLayers = layers.ToArray()
  };
});

//var options = ApplicationOptions.EditingOptions;
QueuedTask.Run(() =>
{
  //change the segment symbol primary color to green and
  //width to 1 pt
  var segSymbol = options.GetSegmentSymbolOptions();
  segSymbol.PrimaryColor = ColorFactory.Instance.GreenRGB;
  segSymbol.Width = 1;

  //Are these valid?
  if (options.CanSetSegmentSymbolOptions(segSymbol)) {
    //apply them
    options.SetSegmentSymbolOptions(segSymbol);
          }
});

//var options = ApplicationOptions.EditingOptions;
QueuedTask.Run(() =>
{
  var def_seg = options.GetDefaultSegmentSymbolOptions();
  options.SetSegmentSymbolOptions(def_seg);
});