Color a specific word in a long text in c# [closed]

private void  txt_TextChanged(object sender, EventArgs e)
        {
            this.CheckKeyword("passed", Color.Purple, 0);
            this.CheckKeyword("failed", Color.Green, 0);
        }
private void CheckKeyword(string word, Color color, int startIndex)
    {
        if (this.txt.Text.Contains(word))
        {
            int index = -1;
            int selectStart = this.Rchtxt.SelectionStart;

            while ((index = this.txt.Text.IndexOf(word, (index + 1))) != -1)
            {
                this.txt.Select((index + startIndex), word.Length);
                this.txt.SelectionColor = color;
                this.txt.Select(selectStart, 0);
                this.txt.SelectionColor = Color.Black;
            }
        }
    }