WPF - How to get a cell from a DataGridRow?

I have a data-bound DataGrid with alternating row background colors. I would like to color a cell differently based on the data it contains. I have tried the solution suggested by this thread

http://wpf.codeplex.com/Thread/View.aspx?ThreadId=51143

But,

DataGridCellsPresenter presenter = GetVisualChild(row)

always returns null.

I am using

    public static T GetVisualChild<T>(Visual parent) where T : Visual
    {
        T child = default(T);
        int numVisuals = VisualTreeHelper.GetChildrenCount(parent);
        for (int i = 0; i < numVisuals; i++)
        {
            Visual v = (Visual)VisualTreeHelper.GetChild(parent, i);
            child = v as T;
            if (child == null)
            {
                child = GetVisualChild<T>(v);
            }
            if (child != null)
            {
                break;
            }
        }
        return child;
    }

But VisualTreeHelper.GetChildrenCount() of a DataGridRow always returns 0. I have verified that DataGridRow is not null and has been populated with data already. Any help is appreciated.

Thanks.