JS snippet that add/removes a class when the placeholder is selected:
<pre class="lang:js decode:true">var $selects = $('.gfield_select'); 

$selects.each(function () {
    updateSelect(this);
});

$selects.on('change', function() {
    updateSelect(this);
});

function updateSelect(select) {
    var $select = $(select);
    if ( $select.find(":selected").hasClass('gf_placeholder') ) {
        $select.addClass("placeholder_selected");
    } else {
        $select.removeClass("placeholder_selected");
    }
}
</pre>
Example CSS:
<pre class="lang:css decode:true">.gfield_select {
    option {
        color: $body-color;
        
        &amp;.gf_placeholder {
            color: $placeholder-color;
        }
    }
    
    &amp;.placeholder_selected {
        color: $placeholder-color;
    }
}
</pre>