Getting a whole design out of two inks

The constraint that improves the work

A great deal of real print is one or two colours: letterheads, forms, invoice books, rubber stamps, screen-printed bags and shirts, newspaper advertising, cheap flyers, packaging for small producers. It is cheaper, it turns round faster, and the shops that do it are everywhere.

It is also where design gets interesting, because the constraint forces decisions.

The range available from one ink

One plate does not mean one tone. It means one hue at any percentage.

• 100%: the solid.
• 60 to 70%: reads as a distinctly lighter version, still confident.
• 30 to 40%: a background tone, still clearly the colour.
• 10 to 15%: a tint panel. Below this, dot gain and the press’s minimum dot make it unreliable.
• Reversed: the paper showing through a solid. This is your second “colour”, free.

Five values from one plate. Most one-colour work uses two of them and looks impoverished for no reason.

Two inks make more than two colours

With two plates you get: each ink’s tint range, plus overprinting: one ink printed on top of the other, which mixes.

A warm red and a deep blue overprinted give a dark plum. Tinted against each other (100% blue over 40% red, 40% blue over 100% red) give two more. A two-colour job can honestly show eight or nine distinguishable colours.

Two cautions, both real:

• The overprint colour is not predictable from the screen. Transparent ink over transparent ink is not the same as a screen’s multiply blend. Ask the shop to run a small overprint chart, or use one they already have.
• Ink sequence matters. Blue over red and red over blue are not the same colour, because the first ink down is partly covered rather than fully transparent. The press decides sequence; ask.

Duotone for a photograph

A photograph in one ink is a monotone: grey levels mapped onto one colour. Flat, and it loses shadow depth because one ink cannot reach a deep black.

A duotone maps the image onto two inks with different tonal curves: typically black carrying the shadows and a colour carrying the midtones. The result has visibly more depth than either alone, and it is a genuine technique rather than a fallback.

Free path, in GIMP: convert to greyscale, then Colours ▸ Map ▸ Gradient Map with a two-colour gradient, or use Colours ▸ Curves on separate channels after converting to RGB. Photopea has a Duotone mode under Image ▸ Mode. Krita has per-channel curves. None of these is as controllable as Photoshop’s Duotone dialog with its per-ink curves, and all of them produce a usable result.

For a screen-printed job, remember that the two “inks” here become two screens, and the halftone angles must differ or you get moiré.

Designing for it from the start

Decide the colour count before you draw. Retrofitting a four-colour design into two inks is a redesign, not a conversion.

Practical rules:

• Build everything from flat fills and tints of the two inks, named as swatches, from the first minute.
• Set up your document with exactly two spot swatches and delete the rest. If you cannot pick a colour, you cannot use it.
• Work in greyscale first for the tonal structure, then assign the two hues. The structure is what makes it work.
• Keep a printed tint chart of your two inks beside you.

The economics, so you can advise a client

A shop’s quotation for a short run is dominated by setup, not by ink. Each additional plate adds plate cost, make-ready time, and a wash-up between jobs. On a 1,000-piece run, going from two colours to four can add a third to the price; on a 100,000-piece run the difference is small because setup is amortised.

So the honest advice to a small client is: short run, few colours; long run, colours are cheap. Being able to say that, with the reason, is worth more to them than any amount of styling.

Digital presses change the arithmetic, and it is worth knowing which kind of machine a job is going on. A toner or inkjet digital press has no plates at all, so four colours cost the same as one and a run of fifty is viable. What it cannot do is spot inks, metallics, or the particular density of a solid litho colour. So the rule inverts: on digital, use all four colours freely and forget about plate counts; on litho, count the plates before you draw.

Ask which press before you design. “Is this digital or offset?” is one line, and it changes the design brief rather than just the quotation.