Tom Arah investigates how to bring your colour under control

In these days of full-colour displays, budget colour lasers and photo-quality inkjets it might seem that computer-based colour handling is hardly an issue. But when you need accurate and consistent colour it’s another matter entirely...

Colour Spaces: Understanding the Problem

To understand why, it’s necessary to go back to first principles. The eye perceives colour as a response to different wavelengths of light hitting the retina and stimulating its photoreceptor cells or “cones”. Humans have three types of cone each primarily receptive to a different wavelength – long (L), medium (M) and short (S) - and different wavelengths produce different combinations of response which are perceived as different colours within the full visible spectrum between ultra-violet and infra-red.
This underlying framework provides the key to how we can go about mechanically reproducing the perception of colour. By combining different mixes of Red (R), Green (G) and Blue (B) light broadly corresponding to the L, M and S cones, it’s possible to reproduce a particular colour so that a combination of 100% red and 100% green produces the impression of yellow. By using 8-bits to specify 256 varying intensities of light that each of the red, green and blue phosphors in a computer display emit, it’s possible to reproduce no less than 16-million colours (256 x 256 x 256).
This “additive” RGB mixing model is ideally suited for computer displays, and for scanners and digital cameras, but print works very differently. Rather than emitting light, inks (and toner and dyes) absorb certain wavelengths and reflect others. However, by mixing the “subtractive” colour primaries Cyan (C), Magenta (M) and Yellow (Y) you can control the wavelengths that eventually hit the eye with cyan ink, for example, absorbing red and reflecting blue and green. Throw in a separate BlacK (K) ink for handling solid black (particularly useful for text) and you have the traditional CMYK “process print” system that underpins all commercial print.
This might all sound relatively straightforward. By using the RGB additive primaries and the CMY subtractive primaries, and mapping between them, presumably we can accurately and consistently display and print the full visible spectrum. Absolutely not. To begin with, the wavelengths emitted by the RGB phosphors in a computer display only roughly approximate to the responsivity of the LMS cones so that the full range or “gamut” of colours that the RGB model is capable of describing is only a subset of the full visible “colour space” – many visible colours can’t be reproduced onscreen.
This mismatch is fundamental but in practical terms pales in comparison to the mismatch between the RGB and CMYK colour spaces – not surprising really when you consider that CMYK is trying to precisely control the combination of reflected light through the very imperfect medium of ink. While the majority of colours are reproducible and convertible between models there are whole areas (nearer the respective primaries where colours are most saturated) where this just isn’t possible – whole swathes of colours can be displayed but not printed and, to a lesser extent, vice versa. This explains why you’ll often be disappointed when you compare the flat, matt colours in a commercial print run to the vivid onscreen hues. It also explains the benefits and attractions of printing with additional Pantone inks and of additional inkjet cartridges which extend the printable colour space and so offer much better RGB reproduction (that’s why modern inkjets are effectively treated as RGB devices).
These core differences between colour spaces are only the beginning of our troubles. To produce reliable and accurate colour on screen and in print we need to be able to reliably translate between colour spaces and here we hit another fundamental problem: there isn’t one display colour space and one print space to work with - instead there are any number! The reason is simple. Each colour space is “device-dependent” so that the gamut of colours that a particular monitor or printer can produce depends entirely on its particular combination of phosphors or inks and how they interact with each other. This is why when you walk into a TV showroom you’ll see that the colour on each device is radically different.
In fact it’s even worse than this as changes to brightness, contrast and colour settings, or a different batch of ink or paper, mean that exactly the same hardware device can and will output the same colours differently. And to top it all whether you view the end colours in natural or artificial light will also effect the final perception of colour. Ultimately then the apparent precision of a numerical colour reading such as R:38 G:66 B:194 is very misleading: yes the colour will be a dark blue but the particular shade perceived by the end viewer will vary enormously – unless you take action.