c# how to color each lines in richtextbox

Question

I need to color each line depends on the last char in each string form list. This is my code and it's always make the last line green. What's wrong with it?

List<string> plik = File.ReadAllLines(path).ToList();
        string pom;
        int size = plik.Count;
        richTextBox1.Clear();
        for (int i = 0; i < size; i++)
        {
            richTextBox1.Text += "[" + i.ToString() + "]" + "  " + plik[i] + Environment.NewLine;
            pom =plik[i];
             richTextBox1.Select(richTextBox1.GetFirstCharIndexFromLine(i), richTextBox1.Lines[i].Length);
           // richTextBox1.Select(0, pom.Length);
            if (pom.Substring(pom.Length - 1) == "n")
            {
                richTextBox1.SelectionBackColor = pom.Substring(pom.Length - 1) == "n" ? Color.Red :Color.Red;
            }
            if(pom.Substring(pom.Length - 1) != "n")
            {
                richTextBox1.SelectionBackColor = pom.Substring(pom.Length - 1) != "n"?Color.Green:Color.Green;                  
            }
        }      

Answer 1

just replace

richTextBox1.Text += "[" + i.ToString() + "]" + "  " + plik[i] + Environment.NewLine;

by

richTextBox1.AppendText("[" + i.ToString() + "]" + "  " + plik[i] + Environment.NewLine);

Answer 2

Append the text instead of modifying it in whole. Using += will replace the entire string and hence you'll lose the set color each time. Use AppendText instead.

Also you can remove the unnecessary if s in your code. This should work:

for (int i = 0; i < size; i++)
{
    richTextBox1.AppendText("[" + i.ToString() + "]" + "  " + plik[i] + Environment.NewLine);
    richTextBox1.Select(richTextBox1.GetFirstCharIndexFromLine(i), richTextBox1.Lines[i].Length);
    richTextBox1.SelectionBackColor = plik[i][plik[i].Length - 1] == 'n' ? Color.Red : Color.Green;
}