Change background of a grouped UITableView

You additionally need to disable the background view of _tableView:

[_tableView setBackgroundView:nil];
 _tableView.backgroundColor = [UIColor redColor];

No need to add new view to backgroundView. This is working for me in iOS6.

What I usually do with grouped UITableViews is set the background color to clear, and the set that pattern image to the parents view.

self.parentViewController.view.backgroundColor = [UIColor colorWithPatternImage:[UIImage imageNamed:@"tableBG.png"]];
self.tableView.backgroundColor = [UIColor clearColor];

why don't you set the tableView.backgroundView? you can alloc an image view withe the specified image and pass it to the background view instead of setting the background color.