Colour management for print: sRGB, CMYK, ICC profiles and proofing

A screen makes colour with light and a press makes colour with ink, so something has to decide how a screen colour is mapped to ink. The ICC (International Color Consortium) FAQ notes that for scanned natural photographic images the colour range of a print or display is often lower than the original [2], and that CMYK has a different colour gamut from a display [2]. Colour management is the system that makes that decision in a controlled way instead of leaving it to each device.

Three things must hold: the file carries a known colour space, the output device has a good profile, and the rendering intent suits the job, with devices kept consistent [2]. The rest of this guide takes the pieces one at a time.

These terms are often mixed up, so keep them apart.

• Standard colour space (sRGB is the example here): a defined colour encoding your artwork is saved in. The ICC FAQ says sRGB was designed as a standard colour space for consumer and internet use [2].
• Device profile: a profile for one specific device, such as a display, a printer or a press. The FAQ describes the two main types as source (input) and destination (output) profiles [2].

In plain terms: a standard colour space travels with your artwork, and a device profile describes where the artwork is going. Colour management joins the two by converting the image from its source profile to the output profile of the print process.

The FAQ says profiles essentially consist of tables of data that relate device co-ordinates to those of the standard colour space defined by the ICC, and that conforming profiles can be exchanged and correctly interpreted by other users [2]. A profile describes a device or colour space; it is not a filter applied to your artwork. Profiles take dot gain into account, so one built from a measured target already contains it [2].

For CMYK, the output profile is built from measured data. The Printing United Alliance describes a CMYK profile as a lookup table between CMYK values and the L*a*b* colour space, and calls profiles the backbone of colour management activities [4]. It names the IT8.7/5 chart, also called TC1617, as the most popular vendor-independent test chart. It has 1,617 CMYK patches [4]. Total area coverage, GCR/UCR and black point settings are chosen when the profile is created [4]. Without a profile there is no way to tell what separation parameters (dot gain, black generation, UCR, GCR) were applied to a CMYK file [2].

The ICC FAQ describes sRGB as a standard colour space that many devices assume [2]. The ICC lists sRGB2014.icc among its v2 sRGB profiles. It has a black point scaled to zero and a preferred rendering intent of perceptual, and it was revised in February 2015 to bring the black point tag into line with current standards [1]. The ICC describes the v4 Preference profile as a v4 replacement for commonly used v2 sRGB profiles. It says the v4 profile should ideally not be combined with v2 profiles, and points to its White Paper 26 if that is unavoidable [1].

The FAQ also notes that sRGB assumes a D65 white point, so a screen used for judging sRGB colour should use D65 [2]. The guides do not promise that an sRGB file will match in print. Our advice: name your profile and keep it consistent through the workflow.

When artwork moves from RGB to CMYK, some screen colours fall outside what the ink can reproduce. A rendering intent sets how that gap is handled. The ICC FAQ defines it as how the gamut of colours achievable on one medium is modified when reproduced on a medium with a different gamut [2]. Scanned natural photographic images reproduced on prints or displays will usually use a perceptual rendering, although a colorimetric rendering may suit high-gamut printing, while proofing (simulating one device on another) calls for a colorimetric intent when there are no gamut mismatches [2].

One more point from the FAQ: if you send RGB data to several printers, it would not be surprising that each returns a different result, because they have no guidance on how to separate it. This is one of the reasons the FAQ says many people prefer to supply CMYK separated data [2].

The table after this section sets out the intents as the ICC guides describe them. It is a summary, so read the ICC pages for the full wording.

A proof shows what the print should look like before the full run. There are two kinds.

Soft proof. This simulates the print on a display. The ICC FAQ describes the principle as multi-stage. The embedded RGB profile is combined with the forward perceptual CMYK profile, then the inverse colorimetric CMYK profile maps the colours into CIELAB but constrained to the CMYK gamut, and finally the display profile is added. Out-of-gamut colours are clipped, so the display shows the colours within the CMYK gamut [2].

Hard proof. This is a physical print. For a contract proof, the profile manages the proofing system so its output lines up with the target press, and it supplies gamut boundary information so an inkjet proofer does not create colours outside the press gamut [4]. The FAQ adds that many people use inkjet printers for proofing, and that this can be effective if the printer has a good colour gamut, a suitable paper is found and colour management is used, providing you do not need a traditional halftone structure on the proof [2].

A soft proof needs no physical sample, but it is only as reliable as the display and its profile, and the FAQ says all devices should be consistent and calibrated if necessary [2]. A hard proof is a physical print you can place beside a sample.

Viewing light. sRGB assumes D65 (see the sRGB section). The FAQ author adds that he knows of no major region using anything other than D50 as the graphic arts standard, in an answer from the older ICC archive [2]. Our own inference is that a screen set to D65 and a print approved under D50 light may not look the same, so ask your printer which light it approves under. The table after this section compares the two kinds of proof.

Use this checklist, and ask the printer which items it needs.

1. The artwork file, with its profile embedded. Embedding keeps the profile permanently associated with the file through later processing and file handling [3]. It does not make sure the profile is used, because that depends on whether the application is colour management aware and its settings [3].
2. The name of the colour space. The ICC FAQ warns that if a file arrives with no input profile the receiver does not know how to convert it accurately and has to guess [2].
3. CMYK only if you know the target. If you convert to CMYK yourself, you need to know what the printer wants in terms of ink colour and dot gain, or share your profile so it can match your proof [2].
4. A reference proof. For a match to a customer proof, the FAQ advises getting the profile used for that proof, so the printer can repeat what was done and correct it for its own process [2].
5. The printer's written rules. Colour requirements differ between printers, so confirm them in writing. Our canvas resolution and colour guide shows where printers disagree, and the artwork file checklist covers size, format and edges.

Whatever you send, approve a sample before the bulk run. See sampling before a bulk order.

ArtzFolio makes private-label frames, wall art, boards, fabric furnishings and wallpaper in Noida, for businesses and brands only. Samples are paid, and the cost may be reimbursed or adjusted by agreement. Our quote confirms price, minimum order and lead time. Payment is 50% advance, with the balance as agreed, and our shipping term is EXW Noida. We reply within 1 business day. Send your artwork, profile name and any reference proof with your enquiry, and see pricing, minimums and lead times or contact us.

1. sRGB profiles (ICC RGB registry), International Color Consortium (ICC). registry.color.org
2. ICC Frequently Asked Questions, International Color Consortium (ICC). archive.color.org
3. ICC White Paper 36: Embedding and referencing ICC profiles, International Color Consortium (ICC). archive.color.org
4. Proof-to-Press Match Part 2: The Role of the Profile, Printing United Alliance. printing.org