How Do I Change Back Color Of A Datagrid View Row Based In A Particular Condition?

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C#

WinForms

I am making a program in c# that takes student roll number, name, marks and grade. the first three fields are inserted by the user where as the grade is calculated automatically by the program based upon student's marks.I tried using
dataGridView1.CurrentRow.DefaultCellStyle.BackColor = Color.Yellow;
But it changes the color of the next row.
this is my code for the calculate grade function:

private string cal_grade()
       {
           string grade = "";
           marks = Int32.Parse(markstb.Text);
           if (marks >= 87 && marks <= 100)
           {
               dataGridView1.CurrentRow.DefaultCellStyle.BackColor = Color.Yellow;
               grade = "A";
               a++;
               Albl.Text = a.ToString();

           }
           else if (marks >= 80 && marks <= 86)
           {
               grade = "B+";
               bp++;
               Bpluslbl.Text = bp.ToString();
           }
           else if (marks >= 72 && marks <= 79)
           {
               grade = "B";
               b++;
               Blbl.Text = b.ToString();
           }

           else if (marks >= 66 && marks <= 71)
           {
               grade = "C+";
               cp++;
               Cpluslbl.Text = cp.ToString();
           }

           else if (marks >= 61 && marks <= 65)
           {
               grade = "C";
               c++;
               Clbl.Text = c.ToString();
           }
           else if (marks >= 50 && marks <= 60)
           {
               grade = "D";
               d++;
               Dlbl.Text = d.ToString();
           }
           else if (marks <= 50)
           {
               grade = "F";
               f++;
               Flbl.Text = f.ToString();
           }
           return grade;
       }