You can specify the color of the line when you create the DirectionsRenderer, using the optional DirectionsRendererOptions struct.
This works for me, simply changing the line where the DirectionsRenderer object is created:
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="initial-scale=1.0, user-scalable=no">
<meta charset="utf-8">
<title>Directions service</title>
<link href="https://google-developers.appspot.com/maps/documentation/javascript/examples/default.css" rel="stylesheet">
<script src="https://maps.googleapis.com/maps/api/js?v=3"></script>
<script>
var directionsDisplay;
var directionsService = new google.maps.DirectionsService();
var map;
function initialize() {
directionsDisplay = new google.maps.DirectionsRenderer({
polylineOptions: {
strokeColor: "red"
}
});
var mapOptions = {
zoom:7,
mapTypeId: google.maps.MapTypeId.ROADMAP,
center: {lat: 41.850033, lng: -87.6500523}
}
map = new google.maps.Map(document.getElementById('map-canvas'), mapOptions);
directionsDisplay.setMap(map);
}
function calcRoute() {
var start = document.getElementById('start').value;
var end = document.getElementById('end').value;
var request = {
origin:start,
destination:end,
travelMode: google.maps.DirectionsTravelMode.DRIVING
};
directionsService.route(request, function(response, status) {
if (status == google.maps.DirectionsStatus.OK) {
directionsDisplay.setDirections(response);
}
});
}
google.maps.event.addDomListener(window, 'load', initialize);
</script>
</head>
<body>
<div id="panel">
<b>Start: </b>
<select id="start" onchange="calcRoute();">
<option value="chicago, il">Chicago</option>
<option value="st louis, mo">St Louis</option>
<option value="joplin, mo">Joplin, MO</option>
<option value="oklahoma city, ok">Oklahoma City</option>
<option value="amarillo, tx">Amarillo</option>
<option value="gallup, nm">Gallup, NM</option>
<option value="flagstaff, az">Flagstaff, AZ</option>
<option value="winona, az">Winona</option>
<option value="kingman, az">Kingman</option>
<option value="barstow, ca">Barstow</option>
<option value="san bernardino, ca">San Bernardino</option>
<option value="los angeles, ca">Los Angeles</option>
</select>
<b>End: </b>
<select id="end" onchange="calcRoute();">
<option value="chicago, il">Chicago</option>
<option value="st louis, mo">St Louis</option>
<option value="joplin, mo">Joplin, MO</option>
<option value="oklahoma city, ok">Oklahoma City</option>
<option value="amarillo, tx">Amarillo</option>
<option value="gallup, nm">Gallup, NM</option>
<option value="flagstaff, az">Flagstaff, AZ</option>
<option value="winona, az">Winona</option>
<option value="kingman, az">Kingman</option>
<option value="barstow, ca">Barstow</option>
<option value="san bernardino, ca">San Bernardino</option>
<option value="los angeles, ca">Los Angeles</option>
</select>
</div>
<div id="map-canvas"></div>
</body>
</html>
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