colour separation
Colour separation is splitting a page into one drawing per ink, because a press lays its inks down one at a time.
In more detail
A press does not print a colour page in one go. Each ink is laid on the sheet in a pass of its own, from a plate of its own, and the colours you see are the inks lying over one another. Separating is working out what belongs on each of those plates: a photograph of a red apple puts a great deal on two of them, next to nothing on a third.
Four plates carry the inks every printing works has to hand. Any ink it keeps besides those — a firm's own blue, a gold, a varnish — gets a plate to itself, and the job then runs on five or six. Nothing about this is a matter of taste: which plate a mark lands on follows from the colour that mark is said in, and from nothing else.
What it does on a real document
A file says its colours in one of two ways, and the difference is the whole of the subject. It can give the four amounts of the usual inks, in which case the separating is already done and the plates follow straight from the numbers. Or it can name an ink and give one figure — how much of that ink goes down — and the name is what tells the printing works which plate those marks belong on.
A named ink may not exist where the file is being looked at: on a screen, or on a press that does not carry it. So the file carries, beside each name, a stand-in mixed from the usual four. That is what you see in the samples of the gallery, and it is why they can be looked at on a screen at all — the plate is what the press gets.
Nothing else in the document has to be arranged around it. A named ink reaches a table's rule, a cell, a heading or a run of ordinary text exactly as an everyday colour does.
Where it comes from, and why
Colour printing has been separated since the middle of the nineteenth century, when a print shop cut one stone per colour and a craftsman drew each separation by hand, guessing what the inks would make of one another once they were laid over each other.
Photography through coloured filters took that job over at the end of that century: one plate photographed through each filter, which is where the four usual inks come from. A PDF names what the filters used to find, and a machine works out the rest.
See it on a finished document
Inks on plates of their own Several plates at once, each at its own strength The rules behind this word, and the texts they are read from