I've just been building one of the new Airfix Kates and was impressed by the fine, distinct panel lines and wondered what I could do to accentuate them without making the plane look like a draughtsman's plan.
Idling about in my local Arts and Crafts shop, as one does, I came across the Royal Talens Ecoline line. It's watercolour ink in a bottle, and comes in a useful range of colours including various greys and browns. I used the warm grey with a touch of liquid soap to change the surface tension, and used a broad brush to give the Kate underside a liberal wash with it. An hour later, it was dry. I then used dampened cotton swabs to remove the excess, and the panel lines came out a treat. Experimentation showed it can smudge nicely too to leave airflow streaks.
I read the stuff becomes active again if it gets wet, so I carefully sealed it all under a clear coat.
Unfortunately, you'll have to take my word for it because I am having distinct bother with photographic sites!
Now to work out which inks go with which camouflage types...