System.Drawing.Color.FromArgb(144,238,255);

if(dg.Items[i].Cells[iRow].Text == "没完成")
    {
     dg.Items[i].BackColor=System.Drawing.Color.FromArgb(144,238,255);
    }
    if(dg.Items[i].Cells[iRow].Text == "已完成")
    {
     dg.Items[i].BackColor=System.Drawing.Color.LightGreen;
    }