function createMarker(latlng,html,type) {

		var typeAIcon = new GIcon(baseIcon);
		typeAIcon.image = "http://maps.google.com/mapfiles/ms/icons/blue-dot.png";

		var typeBIcon = new GIcon(baseIcon);
		typeBIcon.image = "http://maps.google.com/mapfiles/ms/icons/red-dot.png";

		var typeCIcon = new GIcon(baseIcon);
		typeCIcon.image = "http://maps.google.com/mapfiles/ms/icons/yellow-dot.png";


		// Set up our GMarkerOptions object
		if (type=='A') {
			markerOptions = { icon:typeAIcon };
		}
		if (type=='B') {
			markerOptions = { icon:typeBIcon };
		}	
		if (type=='C') {
			markerOptions = { icon:typeCIcon };
		}
		
        var marker = new GMarker(latlng, markerOptions);
        GEvent.addListener(marker, "click", function() {
          marker.openInfoWindowHtml(html);
        });
	
		map.addOverlay(marker);
      }

function initialize() {
.
.
.
     VISmarker = new GMarker(new GLatLng(53.98, 13.5));
     map.addOverlay(VISmarker); 
.
.
.
}

	function toggleVisability(markertotoggle) {
		if (markertotoggle.isHidden())
			markertotoggle.show();
		else
			markertotoggle.hide();
	}

 <p onclick="toggleVisability(VISmarker);">Toggle Testmarker</p>