Change Polyline Color Dynamically

I have a web app that will draw a polyline for each user (tracks movement), and I'd like to incorporate some functionality that allows the web app user to 'focus' on a certain user by changing the color of the polyline. It will have to first change all the polylines to red, and then change the selected polyline to blue. I think this is best to avoid focusing on one line, then trying to focus on another and having them both blue.

I'm really not sure how to implement this, but I have functionality that returns a user id when the name is pressed. I just need to iterate over each object (each user's polyline) to change them to red first, then change the specific one to blue. Here is some code below. This is a condensed version of my code.

function User(id) {
this.id = id;

this.locations = [];

this.mark = 0;

this.getId = function() {
    return this.id;
};

this.addLocation = function(latitude, longitude) {
    this.locations[this.locations.length] = new google.maps.LatLng(latitude, longitude);        
};
var polyline;
this.drawPolyline = function(loc) {
        polyline = new google.maps.Polyline({
        map: map,
        path: loc,
        strokeColor: "#FF0000",
        strokeOpacity: 1.0,
        strokeWeight: 2
        });
    polyline.setMap(map);
};

this.removePolyline = function() {
    if (polyline != undefined) {
        polyline.setMap(null);
        }
    }
this.get_user_info = function(user_id) {

var datastr = 'id=' + user_id;
$.ajax({    
    type: "POST",
    url: 'user_api.php',  
    data: datastr,      
    dataType: 'json',                   
    success: function(data){
        var phone_id = data[0];
        var leftDiv = document.createElement("div"); //Create left div
        leftDiv.id = "left"; //Assign div id
        leftDiv.setAttribute("style", "float:left; width:66.5%; line-height: 26px; text-align:left; font-size:12pt; padding-left:8px; height:26px;"); //Set div attributes
        leftDiv.style.background =  divColor;
        //user_name = document.createTextNode(fullName + ' '); //Set user name          
        a = document.createElement('a');
        a.href ="javascript:setFocus('" + phone_id + "');";
        a.innerHTML = fullName + ' ';
        leftDiv.appendChild(a);
      }
    });
  }
}

function setFocus(phone_id) {
    alert(phone_id);
}
function Users() {
this.users = {};

this.createUser = function(id) {
    this.users[id] = new User(id);
    return this.users[id];
};

this.getUser = function(id) {
    return this.users[id];      
};

this.removeUser = function(id) {
    var user = this.getUser(id);
    delete this.users[id];
    return user;
};
}

var users = new Users();

Currently you are not storing the polyline inside the User-object, you should first do it to make the line accessible later:

this.drawPolyline = function(loc) {
        this.polyline = new google.maps.Polyline({//<--note the this
        map: map,
        path: loc,
        strokeColor: "#FF0000",
        strokeOpacity: 1.0,
        strokeWeight: 2
        });
    this.polyline.setMap(map);
};

now you will be able to higlight a line:

Users.prototype.highlightLine=function(id)
{
  for(var k in this.users)
  {
    this.users[k].polyline.setOptions({strokeColor:(id===k)?'blue':'red'});
  }
}

//use it
users.highlightLine(5)//will highlight the line for user with id 5