Server-Side:

 
protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack)
            {
                Map1.MapBackground.BackgroundBrush = new GeoSolidBrush(GeoColor.FromHtml("#E5E3DF"));
                Map1.CurrentExtent = new RectangleShape(-180, 90, 180, -90);
                Map1.MapUnit = GeographyUnit.DecimalDegree;

                WorldMapKitWmsWebOverlay worldMapKitOverlay = new WorldMapKitWmsWebOverlay();
                Map1.CustomOverlays.Add(worldMapKitOverlay);

                ContextMenu menuOnEventLayer = new ContextMenu("area", 200);
                ContextMenuItem menu2 = new ContextMenuItem("click me");
                menu2.Click += new EventHandler<ContextMenuItemClickEventArgs>(menu2_Click);
                menuOnEventLayer.MenuItems.Add(menu2);

                Feature feature1 = new Feature(new RectangleShape(-10, 10, 0, -10));
                Feature feature2 = new Feature(new RectangleShape(10, 10, 20, -10));
                Map1.HighlightOverlay.Features.Add("multipolygon1", feature1);
                Map1.HighlightOverlay.Features.Add("multipolygon2", feature2);
                Map1.HighlightOverlay.ContextMenu = menuOnEventLayer;
            }
        }

        void menu2_Click(object sender, ContextMenuItemClickEventArgs e)
        {
            if(Map1.HighlightOverlay.Features[0].GetShape().Contains(new PointShape(e.Location.X, e.Location.Y)))
            {
            Map1.HighlightOverlay.Features[0].Id="1";
            }
           if( Map1.HighlightOverlay.Features[1].GetShape().Contains(new PointShape(e.Location.X, e.Location.Y)))
            {
            Map1.HighlightOverlay.Features[1].Id="2";
            }
        }


Client-Side:


    <script type="text/javascript">
        function OnMapCreating(Map) {
            OpenLayers.Layer.Vector.prototype.preFeatureInsert = function (feature) {

                if (feature.id == "1") {
                    feature.style.fillColor = "#F3AF20"
                }
                else if (feature.id == "2") {
                    feature.style.fillColor = "#F12B20"

                }
            }
        }

    </script>