CellLoaded and RowLoaded Event

[Rever]

HI,I would like change the cell background,so I used CellLoaded and RowLoaded.
but have a issue,the CellLoaded can't change the cell background(RowLoaded can work ).
I don't what happen.
RowLoaded Event

for (int i = 0; i < e.Row.Cells.Count; i++)

{

    GridViewCell mark = e.Row.Cells[i] as GridViewCell;

 

    string markValue = mark.Value as string;

    if (markValue != null)

    {

        List<string> TEST = new List<string>();

        string[] t = markValue.Split(',');

        foreach (string item1 in t)

        {

            if (item1 == "NE")

            {

                e.Row.Cells[i].Background = new SolidColorBrush(Colors.LightGray);

            }

        }

    }

}

CellLoaded Event

void RgvVerify_CellLoaded(object sender, CellEventArgs e)
       {
               GridViewCell mark = e.Cell as GridViewCell;
               if (mark != null)
               {
                   string markValue = mark.Value as string;
                   if (markValue != null)
                   {
                       List<string> TEST = new List<string>();
                       string[] t = markValue.Split(',');
                       foreach (string item1 in t)
                       {
                           if (item1 == "NE")
                           {
                               mark.Background = new SolidColorBrush(Colors.Green);
                           }
                       }
                   }
               }
       }

 

btw:I also try used StyleSelector,but it  can't 

thanks.
rever.