for (GeoJsonFeature feature : layer.getFeatures()) {

    //Initialise polygon style
    GeoJsonPolygonStyle polygonStyle = new GeoJsonPolygonStyle();
    polygonStyle.setStrokeColor(Color.BLACK);

    switch (feature.getProperty("Zipcode")) {
        case "20004":
            polygonStyle.setFillColor(Color.RED);
            feature.setPolygonStyle(polygonStyle);
            break;
        case "20535":
            polygonStyle.setFillColor(Color.BLUE);
            feature.setPolygonStyle(polygonStyle);
            break;
        default:
            polygonStyle.setFillColor(Color.YELLOW);
            feature.setPolygonStyle(polygonStyle);
            break;
    }
}