Thanks Joe,
The way I did this (which in hindsight wasn't the best) was to prime, preshade, paint the light blue. I then masked with wide tamiya tape and marked the lines as best I could against the instructions with pencil. I then used a new hobby knife to cut the tape but this did result in cutting the underlying plastic, especially as the knife quickly bluntened. I then preshaded the remaining bit and painted the dark blue.
I had numerous instances of paint being removed when I took the tape off. I'd never experienced that before, so I'm assuming that's an acrylic vs enamel problem (never had it happen with enamels with no primer).
Next time I do one, I'll cut the tape off the model to avoid damage to it.
Cheers,
Brett