How do I set the backcolor to default?

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C#

i have a code wich creates an array of buttons then it sorts them using the insertion sort algorithm and it colours the buttons green for comparison and red for swapping.
now..why does the buttons remain coloured and how can i set them to default backcolur


this is the swapping

public  void exchange(int[] A, int m, int n)
       {
           string s;
           int temp;
           but[m].BackColor = Color.Red;
           System.Threading.Thread.Sleep(100);
           but[n].BackColor = Color.Pink;
           System.Threading.Thread.Sleep(100);

           temp = A[m];
           s = but[m].Text;
           A[m] = A[n];
           but[m].Text = but[n].Text;
           A[n] = temp;
           but[n].Text = s;

           but[m].Refresh();

           but[n].Refresh();


       }

and this is the sorting

<pre>public  void sort(int[] A)
        {
            int i, j;
            int N = A.Length;

            for (j = 1; j < N; j++)
            {
                for (i = j; i > 0 && A[i] < A[i - 1]; i--)
                {
                    but[i-1].BackColor = Color.Green;
                    System.Threading.Thread.Sleep(100);
                    but[i].BackColor = Color.GreenYellow;
                    System.Threading.Thread.Sleep(100);
                    but[i].Refresh();
                    but[i - 1].Refresh();
                    exchange(A, i, i - 1);
                }
            }
        }

picture with form[^]

What I have tried:

i've tryed the refresh method but that dosen't work.