Fill color of shapes in layer

A rectangle in the background layer OK:

// A Script by JJMack's


// enable double-clicking from Mac Finder or Windows Explorer
#target photoshop // this command only works in Photoshop CS2 and higher


// bring application forward for double-click events
app.bringToFront();


// ensure at least one document open
if (!documents.length) {
  alert('There are no documents open.', 'No Document');
}


// if at least one document exists, then proceed
else {
  main();
}


///////////////////////////////////////////////////////////////////////////////
// main - main function
///////////////////////////////////////////////////////////////////////////////
function main() {
  try {
  var startRulerUnits = app.preferences.rulerUnits;
  app.preferences.rulerUnits = Units.PIXELS; // tell ps to work with pixels
  var Width = activeDocument.width.value;
  var Height = activeDocument.height.value;
  // declare local variables
  var layers = activeDocument.layers;
  activeDocument.activeLayer = layers[layers.length-1] // Target Bottom Layer
  if ( !activeDocument.activeLayer.isBackgroundLayer ) { 
  app.activeDocument.artLayers.add()
  // declare local variables
  var newlayers = app.activeDocument.layers;
  // arrange layers so new layer added is at bottom
  newlayers[0].move(newlayers[newlayers.length-1], ElementPlacement.PLACEAFTER);
  // Make it a Background Layer
  activeDocument.activeLayer.isBackgroundLayer=1
  }
  fillColor = new SolidColor;
  fillColor.rgb.red = Math.random() * 255
  fillColor.rgb.green = Math.random() * 255
  fillColor.rgb.blue = Math.random() * 255
  app.activeDocument.selection.selectAll();
  app.activeDocument.selection.fill(fillColor);
  app.activeDocument.selection.deselect();
  fillColor.rgb.red = Math.random() * 255
  fillColor.rgb.green = Math.random() * 255
  fillColor.rgb.blue = Math.random() * 255
  var selectedRegion = Array(Array(.1 * Width, .1 * Height), Array(.9 * Width, .1 * Height), Array(.9 * Width, .9 * Height), Array(.1 * Width, .9 * Height));
  app.activeDocument.selection.select(selectedRegion);
  app.activeDocument.selection.fill(fillColor);
  app.activeDocument.selection.deselect();
  app.preferences.rulerUnits = startRulerUnits;
  }


  // display error message if something goes wrong
  catch(e) { alert(e + ': on line ' + e.line, 'Script Error', true); }
}