Thanks to this tartan painting tutorial, I learned of the use of Micron Pens for the thin black stripes. When I did a search on TMP about it, I found one thread, but all the comments were positive for its use. After having lunch with my wife today, we stopped in a Michael's and found a 005 Micron Pen. It took less than 10 minutes to do all the black stripes on the 30-odd tartans. A couple of them need a little touch up - where my pen stroke was a bit flaky. Otherwise, this is an indispensable tool for any tartan painting arsenal. I highly recommend it.
The piper is painted in the Royal Stewart pattern below: