for (int i = 0; i < dataGridView1.RowCount ; i++)
 {
   for (int j = 0; j < dataGridView2.RowCount; j++)
       {
         if ( dataGridView1.Rows[i].Cells[0].Value.ToString() ==
              dataGridView2.Rows[j].Cells[0].Value.ToString())
            {
               dataGridView1.Rows[i].Cells[0].Style.BackColor =
               Color.Yellow;
               dataGridView2.Rows[j].Cells[0].Style.BackColor =
               Color.YellowGreen;
            }
       }
  }