Gradient Polyline with MapKit ios

I'm trying to trace a route on a MKMapView using overlays (MKOverlay). However, depending on the current speed, I want to do something like the Nike app with a gradient while tracing the route, if the color is changing (for example, from green to orange if a user is driving from 65mph to 30mph).

Here's a screenshot of what I want:

So every 20 meters, I adding an overlay from the old to the new coordinates using:

// Create a c array of points. 
MKMapPoint *pointsArray = malloc(sizeof(CLLocationCoordinate2D) * 2);

// Create 2 points.
MKMapPoint startPoint = MKMapPointForCoordinate(CLLocationCoordinate2DMake(oldLatitude, oldLongitude));
MKMapPoint endPoint = MKMapPointForCoordinate(CLLocationCoordinate2DMake(newLatitude, newLongitude));

// Fill the array.
pointsArray[0] = startPoint;
pointsArray[1] = endPoint;

// Erase polyline and polyline view if not nil.
if (self.routeLine != nil)
    self.routeLine = nil;

if (self.routeLineView != nil)
    self.routeLineView = nil;

// Create the polyline based on the array of points.
self.routeLine = [MKPolyline polylineWithPoints:pointsArray count:2];

// Add overlay to map.
[self.mapView addOverlay:self.routeLine];

// clear the memory allocated earlier for the points.
free(pointsArray);

// Save old coordinates.
oldLatitude = newLatitude;
oldLongitude = newLongitude;

Basically I'm adding a lot of little overlays. Then I would like to create the gradient on this little line drawing, so I'm trying to do so in the overlay delegate:

- (MKOverlayView *)mapView:(MKMapView *)mapView viewForOverlay:(id <MKOverlay>)overlay {
    MKOverlayView* overlayView = nil;

    if(overlay == self.routeLine) {
        // If we have not yet created an overlay view for this overlay, create it now. 
        if(self.routeLineView == nil) {
            self.routeLineView = [[[MKPolylineView alloc