We get asked that question frequently. So here's the answer.
On your swimsuit, the lightest colour will dictate the colour of the finished coverstitch thread. This is often nude or white. Why? Well believe it or not, from a distance, a dark colour will stand out more on a light colour around your neck more than a light colour will around your legs.
So this is why suits that have nude on them will have nude thread around the neck. The coverstitch machine does not stop and start well,. So once an open back suit is started, it goes all the way around the suit in a continuous loop. The only exception to that are the leg openings. If we were to stop and start the stitching with every colour change, your suit would not hold together properly. Not only that, on some suits that could be umpteen thread changes, which will increase the time, and therefore, increase the cost.
On some fabric - such as a black & white zebra print, there is just no winning on this one. No matter what we do, black thread shows on white and white thread shows on black. So we pick one, and make it work.