Checkboxes and drop down list

i have some checkboxlist

<input type="checkbox" name="colors" value="blue" />Blue <br/>
<input type="checkbox" name="colors" value="green" />Green<br/>
<input type="checkbox" name="colors" value="yellow" />Yellow<br/>

and also dropdown list

<select id = "ddcolors">
   <option value="blue">Blue</option>
   <option value="green">Green</option>
   <option value="yellow">Yellow</option>
</select>

Depending on which colors are checked - dropdown list should contain corresponding available selections. how can i achive this with clear JS? Thanks.