Free Format Combo (Dropdown) for Multiple Select

How can I do a free format question with the Combo (dropdown menu) option for multiple select, rather than single select?

The following doesn't work:

<SELECT MULTIPLE="multiple" NAME="FF1_COLOR" ID="FF1_COLOR">
<OPTION SELECTED VALUE="">Choose all that apply</OPTION>
<OPTION VALUE="1">Red</OPTION>
<OPTION VALUE="2">Green</OPTION>
<OPTION VALUE="3">Blue</OPTION>
</SELECT>

I am trying out something with the Image Picker JQuery plugin which uses dropdown multiple select .

Thanks.