Just masked the area with some cheap-o masking tape, then carefully cut them out with a hobby knife.

Things I'll do differently next time:
1. Get better masking tape, or even some liquid mask. The cheap masking tape was a real PITA to work with and kept lifting at the edges all the time.
2. Get a smaller/sharper knife. The knife I've got is too big to enable me to easily work in tight corners inside the shell. I had a design worked out for the back half of the shell too, but the knife/tape just wasn't up to the task.
3. Back the light colour with some white, or maybe silver. The yellow didn't turn out as 'vibrant' as I wanted.
4. Take more than a couple of hours to do the job

I'm going to try some of that harlequin paint for my on-road shell. I've seen some photos of shells done with it and it looks great in the sunlight