Color a specific word in a long text in c# -

i have long text contains key word "passed", "failed". wondering how color key work not entire line.

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;             }         }     }