Holographic stock.

Redbubble says its holographic stickers print on a rainbow reflective film without a white underlay, so any white part of a design is transparent and shows the holographic film. The same page advises using black and white deliberately, because black keeps outlines prominent but too much dark colour blocks the effect, and warns that some gradients do not translate while darker ones should be fine. Sticker Ninja likewise says all white or negative space in the artwork prints as pure holographic, while offering a white ink layer that its designers set up. CustomStickers.com says the vinyl has a reflective silver base, so light or pastel colours look more muted and very dark colours lessen the rainbow effect, and that it cannot guarantee exact colour matching. White in the file therefore means film, not white ink, unless the shop adds a white layer behind it. A design that relies on white areas needs rework before it goes on this stock.