change:
$rainbowcart_itemQuantity = floatval("".((isset($_POST["quantity_".$row_rsitemcolors['FirstNumberID'].""]))?$_POST["quantity_".$row_rsitemcolors['FirstNumberID'].""]:"") ."");
to:
$rainbowcart_itemQuantity = floatval("".((isset($_POST["quantity_".$row_rsaddselected['ItemID'].""]))?$_POST["quantity_".$row_rsaddselected['ItemID'].""]:"") ."");