ClusterRenderer.cs

        //Draw a single marker
        protected override void OnBeforeClusterItemRendered(Java.Lang.Object p0, MarkerOptions markerOptions)
        {
            ClusterItem clusterItem = (ClusterItem)p0;

            //Icon for single marker
            if (clusterItem.m_bIsFav)
                markerOptions.SetIcon(BitmapDescriptorFactory.FromResource(Resource.Drawable.marker_fav));
            else
                markerOptions.SetIcon(BitmapDescriptorFactory.FromResource(Resource.Drawable.marker));

            //...
        }

        //Draw a grouped marker
        protected override void OnBeforeClusterRendered(ICluster p0, MarkerOptions markerOptions)
        {
            bool bIsFav = false;
            foreach (ClusterItem itm in p0.Items)
            {
                bIsFav = itm.m_bIsFav;
                break;
            }

            if (bIsFav)
                m_imageviewForMarkerGroup.SetImageResource(Resource.Drawable.marker_cluster_grouped_fav);
            else
                m_imageviewForMarkerGroup.SetImageResource(Resource.Drawable.marker_cluster_grouped);
                
            //...
        }