To change the cell style, a possible solution is to change the column 'styler' definition and then call 'refreshRow' method to refresh specified row.
var dg = $('#dg');
var col = dg.datagrid('getColumnOption','listprice');
col.styler = function(){
return 'background-color:#f6ced8';
};
dg.datagrid('refreshRow',index);