To change the style of the EditOverlay, you can use the FeatureOverlayStyle class where you can set the color and width of the line based features. See the code below where I set the line to green and with a width of 2:
Feature editFeature = new Feature(lineShape);
Map1.EditOverlay.Features.Add(editFeature);
Map1.EditOverlay.TrackMode = TrackMode.Edit;
Map1.EditOverlay.EditSettings.IsDraggable = true;
Map1.EditOverlay.EditSettings.IsReshapable = false;
Map1.EditOverlay.EditSettings.IsResizable = false;
Map1.EditOverlay.EditSettings.IsRotatable = false;
FeatureOverlayStyle featureOverlayStyle = new FeatureOverlayStyle();
featureOverlayStyle.OutlineColor = GeoColor.StandardColors.Green;
featureOverlayStyle.OutlineWidth = 2;
Map1.EditOverlay.Style = featureOverlayStyle;
This affects the style of the EditOverlay when the feature is not being edited. A member of the web development team will follow up on this post for setting the style while the feature is being edited. Thank you.