The recordset that you use to populate a select list will not have any filtering applied to it. to get the proper information into the cart, you must use a recordset that is filtered.
You will need to create a new recordset to act as a look up for the thumbnail. Make note of the name of the color select list, also note which column is being used for the Value, this column will be the Filter Column when creating the new recordset.
Set the filtering options to use the Value Column form the select list equal to the Color select list form element.