http://jsfiddle.net/#save

HTML

<div class="styled-offer">

<select class="form-control offer-select" id="frmMaand" name="type">
    <option value="1" class="disabled-option   selected" hidden selected  >
        <span>Select an option</span>
    </option>
    <option value="value1" >Option 1</option>
    <option value="value2">Option 2</option>
    <option value="value3">Option 3</option>
    <option value="value4">Option 4</option>
</select>

</div>

CSS

option {
  color:#999;
}

    /* NOT working */
    option:selected {
      color:green;
    }

    .styled-offer select.offer-select {
        color:blue;
      }
      .styled-offer select.offer-select option {
        color: #999;
      }