I use black for preshading, on both upper and lower sufaces. Basically, I airbrush most of the panel lines. I freehand it, meaning I don't tape off the lines. I just eyeball it and follow the line as straight as I can.
The tricky part is the overlying coats. You must mist those on gradually, otherwise you can easily apply too much paint and your preshading will disappear.
I'll add this disclaimer: A lot of people don't care for preshading, saying it is unrealistic. Personally, I use it and think, if well done, it's a very useful technique. Unfortunately, I still haven't perfected it myself!
Hope this helps,
Mark