How do i color the line ?

I have this event mouse down:

private void scrollLabel1_MouseDown(object sender, MouseEventArgs e)
        {
            for (int i = 0; i <  ScrollLabel._lines.Length; i++)
            {
                int j = ((int)(i / 3)) * 3;
                colorLine(j);
                colorLine(j + 1);
            }
        }

And the method colorLine:

private void colorLine(int j)
        {
            string i = ScrollLabel._lines[j]; // put here code to color the line
        }

The problem is im not sure how to color the line in the colorLine method how can i do it ?