javascript - get and change style property openlayers

i need to change the property of some style after its created, and i need to change it with knowing only the string of the property name. i am getting string like "fillColor" and value like "blue", and i need to change the style object. how can i do it? i didnt find anything in the api. the style code:

            var selectStyle = new OpenLayers.Style({
            externalGraphic: 'styles/tree_selected.png',
            graphicWidth: 16, graphicHeight: 26, graphicYOffset: -24,
            title: 'tree'
        });