overprint

Overprinting means deliberately printing one ink colour directly over another, rather than leaving a gap between them. It exists to solve a real problem in multi-colour printing: registration error. When a sheet passes through the press multiple times or when different colour stations run in sequence, the separate colours rarely align perfectly. A thin white line, or gap, can appear where two colours should meet. Overprinting fills that gap by extending one colour slightly into the area reserved for the next.

The technique works by printing a darker or more opaque colour on top of a lighter one, or by using transparent inks that optically blend rather than obscure. A red printed over yellow produces orange in the overlap zone; the eye reads it as continuous colour rather than misregistration. In offset lithography, this is controlled at the design stage: the artwork is separated so that one colour extends fractionally beyond its boundary into the next colour's space. The amount of overprint, typically 0.1 to 0.2 millimetres, is specified in the print specifications.

Overprinting introduces its own hazards if used carelessly. Too much overlap can create visible colour shift, darkening, or loss of fine detail. Ink trapping becomes critical: if the wrong ink sits on top, the result is muddy or unpredictable. Some colour combinations refuse to trap at all; an oil-based ink over a water-resistant varnish, for instance, will not bond properly and may flake. The sequence matters. Many shops use the rule that lighter inks print first, and darker inks overprint them.

Spot colour and process colour overprinting

In process printing (CMYK), overprinting is almost always necessary at colour boundaries because registration is never perfect. In spot colour work, designers and printers must decide case by case. A two-colour logo on plain paper might be better handled with precise knockout (no overlap) if the press can hold registration within 0.05 millimetres. A spot colour over a process background often requires overprint to avoid a halo.

Modern design software and RIP systems allow granular control. A designer can specify which separations overprint which others, and by how much. Testing on a colour separation proof is standard practice before committing to a full run. The alternative to overprinting is trapping, which involves shrinking one colour and expanding an adjacent one to create a deliberate thin overlap of only the edge; it is more precise but demands more skilled artwork and is falling out of use as registration tolerances tighten.