White ink is a separate ink channel used to print opaque white on dark or transparent substrates. Standard CMYK assumes a white or light background; on dark or clear materials, color alone is translucent or invisible without a white base.
When white ink matters
Dark vinyl, clear vinyl, and colored substrates need white under color so graphics are visible and saturated. Without white, colors wash out or disappear. White can also be used as a spot layer for design effect (e.g. white type on dark).
How it is printed
White is typically printed first (underbase) or as a spot layer in a dedicated pass. Equipment and RIP must support white; not every printer runs it. File setup must define where white prints (knockout vs overprint).
Before you specify white ink
• Confirm the substrate (dark, clear, or colored).
• Confirm the printer runs white ink and how they want the file (spot layer, etc.).
• Set up the file with white as a separate layer or spot; do not assume the RIP will infer it.
Common mistake
Sending a file built for white paper and expecting vibrant print on dark or clear. Without a defined white layer, the job will look washed out or wrong. Specify white need and file setup with the printer.