jQuery(document).ready(function() {
    // Selects swatch by default if there is only 1 displayed
    jQuery("#your-swatch-parent-class").each(function(){
        var swatches = jQuery(this).find('li'),
            numswatches = swatches.length;
        if (numswatches === 1) {
            jQuery(this).find("span").trigger("click");
        };
    });
});