How to validate size and color selection

Now I'm really confused. I've tried both client and server validation but they're not working. If I don't bind the color and size to the add to cart, how will it get passed on? I'm using the PayPal Standard so I want to pass these options on.

If the item has no color or no size, then I have a hidden form field to pass the string "n/a" so that there is always a form field for color and for size.

Here's an example of the code for color (size is the same):
<!--- //////////////////////// If there is a selection for colors or sizes, display it here --->
<CFIF #getItem.Colors# NEQ "">
<tr>
<td colspan="6" align="left">
<!--- If Color Selection is available, create drop down list box from array --->
<SELECT name="os0" SIZE="1" class="ppform">
<OPTION value="not selected" >Select Color&nbsp;</OPTION>
<CFLOOP FROM="1" TO="#ArrayLen(ColorList)#" INDEX="i" >
<CFOUTPUT>
<OPTION value="#trim(ColorList[i])#">#ColorList[i]# </OPTION>
</CFOUTPUT>
</CFLOOP>
</SELECT>
PLEASE SELECT A COLOR
<CFELSE>
<input name="os0" type="hidden" value="N/A"/>
&nbsp; </td>
</tr>
</CFIF>

There must be some way to validate that a color is chosen if one is available - any other suggestions?