If you use Google Maps API v3 you can use setIcon
e.g.
marker.setIcon('http://maps.google.com/mapfiles/ms/icons/green-dot.png')
Or as part of marker init:
marker = new google.maps.Marker({
icon: 'http://...'
});
Other colours:
Use the following piece of code to update default markers with different colors.
(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_ROSE)