Bands are currently running dynamically in the Banded Tree Layout.When there is no data on the nodes in the band, the band is completely invisible.

Q 1 : How can I make the header part always visible even if the related band is empty? Q 2: how do i change the background colors of the headers individually?

myDiagram.nodeTemplateMap.add("VerticalBands",
  $(go.Part, "Position", 
    {  
      isLayoutPositioned: false,  // but still in document bounds
      locationSpot: new go.Spot(0, 0, 0, 30),  // account for header height
      layerName: "Background",
      pickable: false,
      selectable: false,
      itemTemplate:
        $(go.Panel, "Vertical",
          new go.Binding("opacity", "visible", function (v) { return v ? 1 : 0; }),
          new go.Binding("position", "bounds", function (b) { return b.position; }),
          $(go.TextBlock, 
            { 
              verticalAlignment: go.Spot.Center,
              textAlign: "center",
              background: headerColor,
              wrap: go.TextBlock.None,
              font: "12pt 'Open Sans', Arial, sans-serif",
              stroke: "transparent",
              height: 30,
            },
            
            new go.Binding("text"),
            new go.Binding("width", "bounds", function (r) { return r.width; })),

          // for rectangular bands:
          $(go.Shape,
            { stroke: null, strokeWidth: 0 },
            new go.Binding("desiredSize", "bounds", function (r) { return r.size; }),
            new go.Binding("fill", "itemIndex", function (i) { return i % 2 == 0 ? "#ececec" : "whitesmoke" ; }).ofObject()),

        )
    },
    new go.Binding("itemArray")
  ));

myDiagram.model = new go.TreeModel(nodearray);

}

 // update the background object holding the visual "bands"
var bands = this.diagram.findPartForKey("_BANDS");
  if (bands) {
    var model = this.diagram.model;
    bands.location = this.arrangementOrigin.copy().add(offset);

    // make each band visible or not, depending on whether there is a layer for it
    for (var it = bands.elements; it.next();) {
      var idx = it.key;
      var elt = it.value;  // the item panel representing a band
      elt.visible = idx < layerRects.length;
    }

    // set the bounds of each band via data binding of the "bounds" property
    var arr = bands.data.itemArray;
    for (var i = 0; i < layerRects.length; i++) {
      var itemdata = arr[i];
      if (itemdata) {
        model.setDataProperty(itemdata, "bounds", layerRects[i]);
      }
    }
  }