<body>
<div id="map" style="width: 800px; height: 600px; border: 1px solid #ccc"></div>
<button id="changeColor">Rectangle -> Blue</button>
<script>
    var osmUrl = 'http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png',
            osmAttrib = '&copy; <a href="http://openstreetmap.org/copyright">OpenStreetMap</a> contributors',
            osm = L.tileLayer(osmUrl, {maxZoom: 18, attribution: osmAttrib}),
            map = new L.Map('map', {layers: [osm], center: new L.LatLng(-37.7772, 175.2756), zoom: 15});
