I was just doing this in a program I was writing. I was doing something like @itsmatt but I feel a bit simpler. You are able to just set the Selectioncolor and from that point on the RichTextBox will be that color until you change it to something else. If you are testing every line this seems to work out well and is easy.
if(myString == "Long")
{
richTextBox.SelectionColor = Color.Red;
}
else
{
richTextBox.SelectionColor = Color.Green
}
richTextBox.AppendText(myString);