Different Google Maps marker depending on ACF field

I have a google map that is looping through and custom post type to place location markers based on the Advanced Custom Fields map input.

This is working perfectly. I have then managed to create a custom marker image for all the map points, again working perfectly.

I want to be able to have different map markers depending on if an ACF is ticked or not. For example -

if( get_field('coming_soon') )
{
iconBase + 'purple-flag.png'
}
else
{
iconBase + 'green-flag.png'
}

Here is my existing code, what do I need to do to get this working?

// var
    var latlng = new google.maps.LatLng( $marker.attr('data-lat'), $marker.attr('data-lng') );
    var iconBase = '<?php echo get_template_directory_uri(); ?>/images/';
    var marker_image = new google.maps.MarkerImage(
        iconBase + 'green-flag.png',
         null,
         // The origin for my image is 0,0.
         new google.maps.Point(0, 0),
         // The center of the image is 50,50 (my image is a circle with 100,100)
         new google.maps.Point(0, 40)
    );


// create marker
    var marker = new google.maps.Marker({
        position    : latlng,
        map         : map,
        icon: marker_image
    });