Use a [`ol.StyleFunction()`][1] and get the name attribute of the feature. E.g.:

    // vector layer
    var vector = new ol.layer.Vector({
        source: new ol.source.Vector({
            format: new ol.format.GeoJSON(),
            url: 'https://raw.githubusercontent.com/openlayers/ol3/6838fdd4c94fe80f1a3c98ca92f84cf1454e232a/examples/data/geojson/countries.geojson'
        }),
        style: function(feature, res){
            // replace "Germany" with any country name you would like to display...
    		if(feature.get("name") == "Germany"){
    			return new ol.style.Style({
    				stroke: new ol.style.Stroke({
    					color: 'red',
    					width: 2
    				})
    			});
    		}
    	}
    });


  [1]: http://openlayers.org/en/latest/apidoc/ol.html#.StyleFunction