Floss bodies often get tied on hooks with black nickel finishes, think Atlantic Salmon flies. Bright flosses like this have a tendency to look darker after they’ve been wrapped because they’re not completely opaque. The effect is even worse when the fly gets wet and the floss saturated.
To alleviate this, many tiers use a white underbody beneath the floss. A thread like white UTC 140 Denier works especially well.
After getting the thread started on the hook shank, give your bobbin a healthy clockwise spin, as if you’re looking down on it, to cord up the thread. This will make it more opaque so as you take rearward touching wraps, it will dramatically brighten the black nickel hook shank. At the rear end of the body, give your bobbin a counterclockwise spin to flatten out the thread. Forward touching wraps with the flattened thread will brighten the underbody even further. Give your thread a counterclockwise twist every so often to keep the thread flat as you wrap.
With the white underbody established, even the lightest color flosses, once wrapped, will retain their brilliant light color and not darken when saturated with water.