var drawingManager;
var selectedShape;
var colors = ['#1E90FF', '#FF1493', '#32CD32', '#FF8C00', '#4B0082'];
var selectedColor;
var colorButtons = {};

function clearSelection() {
  if (selectedShape) {
    selectedShape.setEditable(false);
    selectedShape = null;
  }
}

function setSelection(shape) {
  clearSelection();
  selectedShape = shape;
  shape.setEditable(true);
  selectColor(shape.get('fillColor') || shape.get('strokeColor'));
}

function deleteSelectedShape() {
  if (selectedShape) {
    selectedShape.setMap(null);
    // To show:
    drawingManager.setOptions({
      drawingControl: true
    });
  }
}

function selectColor(color) {
  selectedColor = color;
  for (var i = 0; i < colors.length; ++i) {
    var currColor = colors[i];
    colorButtons[currColor].style.border = currColor == color ? '2px solid #789' : '2px solid #fff';
  }

  // Retrieves the current options from the drawing manager and replaces the
  // stroke or fill color as appropriate.
  var polylineOptions = drawingManager.get('polylineOptions');
  polylineOptions.strokeColor = color;
  drawingManager.set('polylineOptions', polylineOptions);

  var rectangleOptions = drawingManager.get('rectangleOptions');
  rectangleOptions.fillColor = color;
  drawingManager.set('rectangleOptions', rectangleOptions);

  var circleOptions = drawingManager.get('circleOptions');
  circleOptions.fillColor = color;
  drawingManager.set('circleOptions', circleOptions);

  var polygonOptions = drawingManager.get('polygonOptions');
  polygonOptions.fillColor = color;
  drawingManager.set('polygonOptions', polygonOptions);
}

function setSelectedShapeColor(color) {
  if (selectedShape) {
    if (selectedShape.type == google.maps.drawing.OverlayType.POLYLINE) {
      selectedShape.set('strokeColor', color);
    } else {
      selectedShape.set('fillColor', color);
    }
  }
}

function makeColorButton(color) {
  var button = document.createElement('span');
  button.className = 'color-button';
  button.style.backgroundColor = color;
  google.maps.event.addDomListener(button, 'click', function() {
    selectColor(color);
    setSelectedShapeColor(color);
  });

  return button;
}

function buildColorPalette() {
  var colorPalette = document.getElementById('color-palette');
  for (var i = 0; i < colors.length; ++i) {
    var currColor = colors[i];
    var colorButton = makeColorButton(currColor);
    colorPalette.appendChild(colorButton);
    colorButtons[currColor] = colorButton;
  }
  selectColor(colors[0]);
}

function initialize() {
  var map = new google.maps.Map(document.getElementById('map'), {
    zoom: 10,
    center: new google.maps.LatLng(22.344, 114.048),
    mapTypeId: google.maps.MapTypeId.ROADMAP,
    disableDefaultUI: true,
    zoomControl: true
  });

  var polyOptions = {
    strokeWeight: 0,
    fillOpacity: 0.45,
    editable: true
  };
  // Creates a drawing manager attached to the map that allows the user to draw
  // markers, lines, and shapes.
  drawingManager = new google.maps.drawing.DrawingManager({
    drawingMode: google.maps.drawing.OverlayType.POLYGON,
    drawingControlOptions: {
      drawingModes: [
        google.maps.drawing.OverlayType.POLYGON
      ]
    },
    markerOptions: {
      draggable: true
    },
    polylineOptions: {
      editable: true
    },
    rectangleOptions: polyOptions,
    circleOptions: polyOptions,
    polygonOptions: polyOptions,
    map: map
  });

  google.maps.event.addListener(drawingManager, 'overlaycomplete', function(event) {
    if (event.type == google.maps.drawing.OverlayType.CIRCLE) {
      var radius = event.overlay.getRadius();
    }
  });
  google.maps.event.addListener(drawingManager, 'overlaycomplete', function(e) {
    if (e.type != google.maps.drawing.OverlayType.MARKER) {
      // Switch back to non-drawing mode after drawing a shape.
      drawingManager.setDrawingMode(null);
      // To hide:
      drawingManager.setOptions({
        drawingControl: false
      });

      // Add an event listener that selects the newly-drawn shape when the user
      // mouses down on it.
      var newShape = e.overlay;
      newShape.type = e.type;
      google.maps.event.addListener(newShape, 'click', function() {
        setSelection(newShape);
      });
      setSelection(newShape);
    }
  });

  // Clear the current selection when the drawing mode is changed, or when the
  // map is clicked.
  google.maps.event.addListener(drawingManager, 'drawingmode_changed', clearSelection);
  google.maps.event.addListener(map, 'click', clearSelection);
  google.maps.event.addDomListener(document.getElementById('delete-button'), 'click', deleteSelectedShape);

  buildColorPalette();
}
google.maps.event.addDomListener(window, 'load', initialize);

#map,
html,
body {
  padding: 0;
  margin: 0;
  height: 100%;
}

#panel {
  width: 200px;
  font-family: Arial, sans-serif;
  font-size: 13px;
  float: right;
  margin: 10px;
}

#color-palette {
  clear: both;
}

.color-button {
  width: 14px;
  height: 14px;
  font-size: 0;
  margin: 2px;
  float: left;
  cursor: pointer;
}

#delete-button {
  margin-top: 5px;
}

<script src="https://maps.google.com/maps/api/js?key=AIzaSyCkUOdZ5y7hMm0yrcCQoCvLwzdM6M8s5qk&libraries=drawing"></script>
<div id="panel">
  <div id="color-palette"></div>
  <div>
    <button id="delete-button">Delete Selected Shape</button>
  </div>
</div>
<div id="map"></div>