Use 100% K where clean registration matters most.
In a conventional CMYK file, 100% K means the object uses the black separation without adding cyan, magenta or yellow. It is usually the sensible starting point for small black text, fine rules, folios, barcodes and other details that must stay crisp. Building the same small characters from several process inks asks multiple plates to align along every edge. Even slight movement can leave a coloured fringe or make the type look heavier.
This is a file decision, not a promise that every black-only area will have the same visual depth. Ink, paper, screening and print process all influence the result. A narrow caption on an uncoated page and a broad black panel on coated board place different demands on the black. Check those vulnerable details at finished size with the small type and fine lines guidance.
Large dark areas may need a composite black agreed for the job.
A rich black combines black with one or more process colours in the same solid area to produce a denser-looking result. It can suit a large cover field, presentation-box panel or oversized display word where black-only ink appears too open for the design. There is no safe recipe to copy into every document. The appropriate build depends on the colour profile, paper or board, press condition, nearby artwork and total ink limit accepted by the receiving workflow.
Ask for the black build when the production route and material are known. A formula used for one coated publication may be unsuitable for an absorbent sheet, a different profile or another printing method. If a dark area continues through a photograph, agree whether the solid and image should share a neutral appearance or remain distinct. Familiar numbers copied from an older job are less dependable than the output conditions described in the RGB to CMYK colour guide.
Black over coloured artwork needs an overprint decision.
Many page-layout workflows overprint the default 100% black swatch automatically. That behaviour can help small black type sit over a coloured background without a fine knockout gap. It can also let the colour below influence a broad black object. A black box intended to conceal an image may therefore behave differently from the same box viewed in an ordinary screen preview.
Do not treat every object that appears black as the same swatch. Imported graphics, unnamed CMYK colours, black tints, transparency effects and duplicated swatches may carry different separation or overprint behaviour. Inspect those objects with the plate-by-plate checks in the overprint and knockout guide. Where concealment is important, state it in the artwork note and inspect the exported PDF with overprint simulation and individual separations visible.
Registration colour is not an extra-rich black.
Registration marks need to appear on every separation so the production team can align and measure the job. Design applications therefore provide a registration swatch that outputs on all plates, including any spot-colour plates. It is intended for production marks, not for body text, logos, backgrounds or decorative rules.
Using registration colour in artwork can create excessive ink and unexpected content on special separations. A logo that should appear only in CMYK might also land on a varnish, white-ink or foil plate. Preflight should flag registration colour inside the finished artwork area, and the separation preview should show whether every named ink contains only what it should.
Judge black through the exported file and the chosen proof.
Software preferences can display all blacks as equally deep on screen, even when their CMYK values differ. Switch to an accurate-black view where available, then inspect the output PDF in a production preview. Toggle the cyan, magenta, yellow and black plates. Small text should show the intended construction; broad solids should use the agreed build; accidental coloured components should not hide inside apparently neutral objects.
A desktop printout can reveal hierarchy and legibility, but it is not a reliable press-colour proof. For a black cover, monochrome image sequence or brand-sensitive dark pack, define what the proof must demonstrate. Use the soft and hard proof comparison to separate a file check from a material and density decision.