dataGridView2_RowPrePaint(object sender, DataGridViewRowPrePaintEventArgs e)//I have used this event

string unit = Convert.ToString(dataGridView2.Columns["exam"]);//One of my datagridview
//column name is exam and in that column cells will be unit1 or unit2 or unit3 or unit 4 or quarterly or halfyearly or yearly
             if (unit == "Unit 1" ||unit == "Unit1" || unit == "Unit2" || unit == "Unit3" || unit == "Unit4")
              {
                  for (int i = 0; i < dataGridView2.Rows.Count; i++)
                  {

                      for (int j = 7; j < dataGridView2.Rows[i].Cells.Count; j++)
                      {
                          if (Convert.ToInt32(dataGridView2.Rows[i].Cells[j].Value) < 13)
                          {
                              dataGridView2.Rows[i].Cells[j].Style.ForeColor = Color.Red;
                          }


                          else
                          {
                              dataGridView2.Rows[i].Cells[j].Style.ForeColor = Color.Green;
                          }
                      }
                  }

              }
              else
              {
                  for (int i = 0; i < dataGridView2.Rows.Count; i++)
                  {

                      for (int j = 7; j < dataGridView2.Rows[i].Cells.Count; j++)
                      {

                          if (Convert.ToInt32(dataGridView2.Rows[i].Cells[j].Value) < 35 )
                             dataGridView2.Rows[i].Cells[j].Style.ForeColor = Color.Red;
                         else
                              dataGridView2.Rows[i].Cells[j].Style.ForeColor = Color.Green;
                      }
                  }
              }