Built, not painted — and why that matters
A painted gradient sits on the surface, so as the nail grows the whole gradient moves down the finger and the fade ends up in the wrong place within about ten days. A built ombré is blended while the acrylic is still workable, so the transition is part of the nail's structure. Three weeks later it still reads as a deliberate fade rather than a mistake.
Building it takes longer because the two powders have to be laid and blended before either sets, one nail at a time, with no way to go back and adjust once they have. That is the entire price difference between an ombré set and a single-color one.