Reference
Colour and ink: what happens between the file and the paper
Ink is a physical substance on a physical surface. Almost everything in this cluster is invisible on a screen and decided in a stylesheet, which is an awkward combination: the choices are yours to make and the consequences only appear on paper.
Where this API stands, first, because it is the question people arrive with. It renders HTML to PDF and the colour in the output is the colour in your CSS, which is RGB. There is no colour-space option, no CMYK mode, no ICC embedding, no ink-limit control and no greyscale conversion. Two options touch colour and neither changes a value: print_background, which is on by default here, and media emulation. Converting a document into inks is a step after the render.
Four colour topics live in the standards cluster rather than here, because that is where conformance owns them: CMYK against RGB and spot colour, ICC profiles and output intents, overprint and transparency flattening, and bleed. Pages below link across to them rather than repeating them.
How the ink lands
Effects that come from ink being a physical substance on a physical surface rather than a value in a buffer. None of them are visible on a screen, and all of them are decided by choices made in the stylesheet.
- Rich black and 100% K
Two different blacks that look identical on screen and behave completely differently on paper: one made from black ink alone, one made by adding the other three inks underneath it.
- Reversed type and knockout
Text knocked out of a solid colour so the paper shows through it, which is the most fragile way to set type in print and the most common on screen.
- Hairlines and thin rules
Rules and borders specified below the width the output device can actually draw, which come out either heavier than asked or not at all.
- Total ink coverage
The sum of all the ink percentages laid in one place, which has a physical ceiling that a colour chosen on a screen knows nothing about.
What happens when the colour is converted
A rendered document is RGB. Almost everything that happens to it afterwards is a conversion, and a conversion is a decision somebody makes: either you make it deliberately or a driver makes it for you.
- Greyscale conversion
What happens when a colour document is printed on a monochrome device, which is most of the time for most documents.
- Tints and dot gain
A percentage screen of a colour, which is printed as a pattern of dots rather than a lighter ink, and comes out darker than specified.
- Duotone and single colour
A document produced with fewer inks than four, either to control cost or because the process only offers one, and what that does to a design built in full colour.
What the output device does to it
The same file on an office laser, a copier and a commercial press produces three different objects. These pages are about which differences matter enough to design around.
- Photocopy legibility
What a document has to be like to still work after it has been through a copier, a scanner or a fax, which is where most operational documents end up.
- Office laser against a press
Two output devices with different resolutions, different colour behaviour and different physical limits, producing two different objects from one file.
- Paper and substrate
The stock the document is printed on, which is the white in every light area and therefore part of every colour on the page.