Why part of the string in the richTextBox is colored in green?

Question

private void ColorRichTextbox(RichTextBox box, string text, Color color, bool appendNewLine = true)  
        {  
            int length = box.TextLength;  
            if (!string.IsNullOrWhiteSpace(box.Text) && appendNewLine)  
            {  
                box.AppendText("\r\n" + text);  
            }  
            else  
            {  
                box.AppendText(text);  
            }  
            box.SelectionStart = length;  
            box.SelectionLength = text.Length;  
            box.SelectionColor = color;  
        }  

Then in button click event the first line in the richTextBox will be light green :

ColorRichTextbox(richTextBoxLogChanges, "Watching Event Started : ", Color.White);  
ColorRichTextbox(richTextBoxLogChanges, DateTime.Now.ToString(), Color.LightGreen, false);  

Then i'm adding another line in yellow :

private void Fsw_Deleted(object sender, FileSystemEventArgs e)  
        {  
            ColorRichTextbox(richTextBoxLogChanges, $"Deleted: {e.FullPath}", Color.Yellow);  
        }  

The yellow line the last char get the light green color of the first line :

In the screenshot in this example the last char S of the css is in light green and it should be in yellow.

Screenshot of the last problem mentioned in my comment/s to the solution :

Answer 1

Check a possible fix:

private void ColorRichTextbox(RichTextBox box, string text, Color color, bool appendNewLine = true)
{
   int length = box.TextLength;
   int sl;
   if (!string.IsNullOrWhiteSpace(box.Text) && appendNewLine)
   {
      box.AppendText("\r\n" + text);
      sl = text.Length + 2;
   }
   else
   {
      box.AppendText(text);
      sl = text.Length;
   }
   box.SelectionStart = length;
   box.SelectionLength = sl;
   box.SelectionColor = color;
}