@YingJanshi: Panel lines! Since there are lots of them running along the body of the model, I used them to mark the border between the blue and white sections. Because the panel lines on this miniature are very clean and straight, they made it easy to paint two different colors with a minimum of mess.
Seriously, I have a lot of praise for this sculpt-it was very clean, easy to assemble and a real pleasure to paint thanks to the crisp, sharp detailing.