1. go to the html screen via the html button
2. In the HTML screen go to the positon where you would like to have you line. And past the code below.
<hr style="color: #FF0000; background-color: #FF0000; height: 1px; border: none;" />
3 Then click on 'Ok' and 'SAVE'.
The result :
Now # FF0000 is basic color red. To change the color you need the correct hex code. You can find that on the internet ( http://www.w3schools.com/html/html_colors.asp)