Colour spaces for printing: sRGB, Adobe RGB, CMYK and profiles

A digital image stores each colour as numbers. A colour space is the agreement that says which real colour those numbers stand for. Without it, the same file can look different on a phone, a laptop and a print. This page explains the colour spaces you are likely to meet when you print a portrait, sRGB, Adobe RGB and CMYK, and the profiles that let a screen and a printer agree. It belongs to the guide to art prints and paper.

What a colour space does

A screen makes colour by mixing red, green and blue light, so most image files store three numbers for each pixel: one for red, one for green and one for blue. A printer makes colour with ink. The Image Permanence Institute at Rochester Institute of Technology says basic inkjet printers use cyan, magenta, yellow and black inks, and some high end printers add light cyan, light magenta, greys and other colours.

The numbers alone do not fix a colour. The 1996 proposal for sRGB, which the World Wide Web Consortium keeps online, describes the problem. Colour described in device terms, such as amounts of ink in CMYK or voltages in RGB, depends on the device. A colour space ties the numbers to measured colour, so different devices can reproduce the same colour from the same file.

A short history of sRGB

sRGB began as a proposal from Hewlett-Packard and Microsoft, published on 5 November 1996. Its authors wanted a single standard RGB colour space for operating systems, device drivers and the Internet, so that images could travel between devices without extra colour data attached to every file. They based it on the screens of the time, cathode ray tube monitors, and defined:

• a set of red, green and blue primaries,
• a tone curve equivalent to a gamma of 2.2,
• reference viewing conditions: a display at 80 candelas per square metre, in a room lit to 64 lux.

The proposal gives 200 lux as its estimate for a typical office or home, and says it chose the dim room figure because allowing for the extra glare of brighter rooms was impractical. The International Electrotechnical Commission later made sRGB an international standard, IEC 61966-2-1, in 1999. The Commission describes it as applying to the encoding and communication of RGB colours in computer systems.

The International Color Consortium, a group of companies that writes the specification for colour profiles, says nearly all web standards call for sRGB. A browser that follows the standards treats RGB colours as sRGB and converts them for the user's display.

sRGB and Adobe RGB

Adobe RGB (1998) is a second common RGB colour space. The International Color Consortium keeps a registry of colour spaces, and its entries show the differences:

A gamut is the full range of colours a device or colour space can hold. The Consortium says the range on a print or a display is often smaller than in the original scene. A wider colour space, such as Adobe RGB, can describe some colours that sRGB cannot.

The Consortium's advice on screens says sRGB may be a suitable choice when the main use is images for the web. It adds that many users prefer a larger working space and accept that a small share of colours may not display correctly, because this lets them use the full range of other devices, such as a printer. It also says most cameras render their images to sRGB or Adobe RGB (1998) and embed the matching profile in the file.

Profiles and colour management

An ICC profile is a data file that describes the colours of one device or colour space. The International Color Consortium says profiles relate the numbers a device sends or receives to a standard colour space, based on measurements defined by the CIE, the International Commission on Illumination. If the camera, the display and the printer each have a profile, software can combine them to keep colours correct from one to the next.

Colour management is the software that does this. The Consortium describes it as a system that converts the data for one device, such as a scanner's RGB, into data for another, such as a printer's CMYK, so the print shows the same colours. Where an exact match is not possible, the result should be a pleasing approximation.

That conversion follows a rendering intent. The Consortium says each profile holds three intents. Photographs are usually reproduced with the perceptual intent, which accounts for a print having a smaller range of colours than the original and adjusts the whole image so it keeps its appearance. The colorimetric intents try for an exact match of the colours that fit, and the saturation intent aims for the most colourful result, which is used for business graphics.

RGB or CMYK for a printer

Print shops and guides sometimes ask for CMYK files. The International Color Consortium answers a common question on this: why does sending CMYK to a printer give a worse result than sending RGB? Its answer is that many printers have only an RGB interface. They take CMYK data, convert it back to RGB, then separate it into their own inks. For such a printer, the Consortium says, it is better to leave the data in RGB. Converting to CMYK only helps if the printer uses CMYK data directly and you have a good profile for that printer.

For a home inkjet printer or a photo printing service, the practical step is to ask what the printer expects. The page on printing a digital download lists the questions to ask a print shop.

Seeing a print on screen first

Soft proofing means using a calibrated screen to preview how an image will look in print. The International Color Consortium's guide to display calibration says a display used this way should be calibrated to a D50 white, the white used in the photography and graphic arts viewing standard ISO 3664, at a brightness of 160 candelas per square metre. It adds that matching the screen's white to the colour of the unprinted paper often helps.

The guide gives other advice for any screen used to judge colour. Keep room lights and sunlight off the screen. Let the screen warm up for 15 minutes before calibrating. Displays change over time, so the Consortium says recalibrating at six month intervals is often enough. It also says that on tablets and phones there is often no colour management at the system level and little control beyond brightness.

Why two screens and two lamps disagree

The same file can look different on two screens because each screen has its own brightness, white and colour settings. The International Color Consortium's calibration guide says displays have controls for brightness, contrast and often colour balance, and that the viewing room changes both the image on the screen and how its colours appear to the eye. The 1996 sRGB proposal added that screens also reflect room light, and that older and less expensive displays could reflect up to 20 percent of it.

A print can change with the lamp as well. The Consortium's answers on colour explain that two colours can match to the eye while being made of different mixtures of light wavelengths, an effect called metamerism. Such a match generally fails when the light shining on the samples changes. So a portrait that matches its screen image under one lamp may look warmer or cooler under another. Judge a test print in the room and under the light where the portrait will hang.

What this means for a royal pet portrait

Purrealism portraits come as a digital download or as an art print on matte or glossy paper, in four sizes from 8 × 10 to 24 × 36 inches, chosen in the portrait studio. If you print the download yourself, keep the file as it is, with any embedded profile, and let the print shop or printer software handle the conversion to ink.

Reds and golds carry portraits in the gallery such as the golden retriever Maharaja in a red jewelled turban on a gold throne. A test print at a small size shows how a printer handles those colours before you pay for a large sheet. For how long those colours last once printed, read the page on lightfastness and fading, and for the pixel count a print needs, the page on print resolution.