To expand upon the accepted answer, you can create custom MarkerImages of any color using the API at http://chart.googleapis.com
In addition to changing color, this also changes marker shape, which may be unwanted.
const marker = new window.google.maps.Marker(
position: markerPosition,
title: markerTitle,
map: map)
marker.addListener('click', () => marker.setIcon(changeIcon('00ff00'));)
const changeIcon = (newIconColor) => {
const newIcon = new window.google.maps.MarkerImage(
`http://chart.googleapis.com/chart?
chst=d_map_spin&chld=1.0|0|${newIconColor}|60|_|%E2%80%A2`,
new window.google.maps.Size(21, 34),
new window.google.maps.Point(0, 0),
new window.google.maps.Point(10, 34),
new window.google.maps.Size(21,34)
);
return newIcon
}