Color of extent in OpenLayers 3

Question

I'm very new to OpenLayers and all the GIS world.

I'm working with rasters (tiff that are converted to postgis sql)

I have this code:

        var feature;
        $.ajax({
            method: "POST",
            url: Routing.generate("getdpageometry"),
            dataType: 'json',
            data: { tipodpa:tipodpa, dpaid:object.id },
            async: false
        }).done(function( obj ) {
            feature = obj.feature;
        }.bind(this));

        var formatWKT = new ol.format.WKT();
        var extentFeature = formatWKT.readFeature(object.extent, {
            dataProjection: 'EPSG:3857',
            featureProjection: 'EPSG:3857'
        });
        var formatGeoJSON = new ol.format.GeoJSON();
        var feature = formatGeoJSON.readFeature(feature, {
            dataProjection: 'EPSG:32719',
            featureProjection: 'EPSG:32719'
        });
        feature.getGeometry().transform('EPSG:32719','EPSG:3857');
        Store.vectorSource.clear();
        Store.vectorSource.addFeature( feature );
        Store.map.getView().fit(extentFeature.getGeometry().getExtent(), Store.map.getSize());

This is part of a code that is showing a map but only a region (defined by an extent in the database) is colored, and adds some blue border to it, like this:

What I need to do is change that blue color, I need it transparent, I only need the border to be blue, and also I would like to change the border color too, so my question is:

Where should I be looking to change those colors?

I'm pretty confused, not sure if it's possible to change it like a parameter, or it's OL3 library default color (I've search into the code, I have changed some blue code colors but nothing happens), or it has to be related with the raster from the database, or where/how...

---

Using the chosen answer, now my code has:

    this.state.vectorLayer = new ol.layer.Vector({
        source: this.state.vectorSource,
        style: new ol.style.Style({
            fill: new ol.style.Fill({  color: 'rgba(0, 0, 0, 0.2)'  }),
            stroke: new ol.style.Stroke({ color: '#FF0000',  width: 2  }),
            image: new ol.style.Circle({
                radius: 7,
                fill: new ol.style.Fill({
                    color: '#ffcc33'
                })
            })
        })
    });

And the map:

Answer 1

See this example from the OpenLayers website and this question on SO:

You have to define a style variable and bind it to your layer:

var stroke = new ol.style.Stroke({
    color: 'rgba(255, 204, 0, 0)',
    width: 1,
});

var style = new ol.style.Style({
    stroke: stroke
});


vector = new ol.layer.Vector({
    source: vectorSource,
    style: style
});  

But your code seems not complete (where is defined the layer?) so I'm not sure if it will fit in without extra modifications.