paypal standard does not include the pp_ pages, those are used by paypal express checkout.
these elements:
<input type="hidden" name="on0_<?php echo $ag_Store_Index+1; ?>" id="on0_<?php echo $ag_Store_Index+1; ?>" value="" />
<input type="hidden" name="os0_<?php echo $ag_Store_Index+1; ?>" id="os0_<?php echo $ag_Store_Index+1; ?>" value="" />
<input type="hidden" name="on1_<?php echo $ag_Store_Index+1; ?>" id="on1_<?php echo $ag_Store_Index+1; ?>" value="" />
<input type="hidden" name="os1_<?php echo $ag_Store_Index+1; ?>" id="os1_<?php echo $ag_Store_Index+1; ?>" value="" />
are used for passing options. the on0_... and on1_.. elements are used for the option name. the os0_... and os1_... are used for the option value.
paypal only allows for 2 options, so you will need to pass Size and Color as one option, use the bindings panel to set the value of the os0_... element to the Size and Color cart columns.
add the other column bindings to the os1_... element. You can add labels to the name elements for the options.