For the racing stripes across the top of this;
I used 1mm masking tape (Goggle "PMA Super Thin Masking Tape").
The lines may look a little offset towards the front, but if you look closely, there is a little bit off detail that the lines had to "climb up" to continue.
I laid the tape down, painted the rest of the Mech and then removed the tape and painted what was under it purple. If you look at a ruler, 1mm is tiny, but it is a lot on a mini... Be warned though: The tape is so thin that it doesn't have a lot of surface area to grab hold. It doesn't take much to have it peel back up. Also, paint tends to accumulate at the edge of the tape. This means you may have to file or scrape it, otherwise you will have an uneven surface.
Some of what I said may make the tape sound like it is more trouble than it is worth, which is not true. Like any technique it just takes a little practice... It really is helpful, not just with pin-stripes, but also with multiple color paint schemes (I'm looking at you Lyran Guard). It can get in tight spaces and around curves in a way that a ruler would have difficulty doing. Granted, it costs a little more than a straight edge and pencil, which you probably have on hand, but it is still worth it.
Caz