Dynamically change the color of a polygon in leaflet?

L.Circle extends L.Path (http://leafletjs.com/reference.html#path), that have method setStyle( <Path options> object ), and you can apply new style as window.circle.setStyle({fillColor: '#dddddd'});

If you are looking for something like this:

const circle = L.circle([lat, lng], {
   style: style,
   onEachFeature: onEachFeature,
});

These options are available for geoJson data ie: L.geojson()..... :D

So, for polygon . Try,

circle.setStyle({
    color: 'red'
});