Andy,
I wouldn't worry too much about the black lines; whilst admittedly in 1:76, I've produced similar on a model and used a permanent Lumocolor black marker. Of course, given that it was 1:76 I used the Fine nib, but I suspect that a Medium in same would work for 1:35; of course, being permanent means just that but the soluble variants would do if you needed to practice first. I found that once I'd added matt varnish (to the Permanent)it looked just fine.
Remember (and I hope I'm not teaching you to suck eggs here) the original would have been hand-painted, probably by the crews. Having seen black paint applied to British military vehicles by use of a mop, I never worry too much about the consistency of a painted line.
Good luck with it all.
Brian