Colour is a slippery topic and one that has always demanded considerable expertise. Because colour doesn't really exist, it is inevitably complex whether we like it or not. Digital colour, where colour transformations are handled in software, should theoretically make matters simpler, but it hasn't worked out that way and mostly it's because of colour space differences.

The advent of digital scanners marked the beginning of the printing industry's move to digital imaging, but even though that first step was taken over 50 years ago colour, digital or otherwise, continues to plague printers. The rising and more ambitious use of it, particularly in the sign-making business, means that the problems won't get simpler. With every new print production convenience that gets added to software, more colour innocents will be able to create colour files of dubious accuracy.

Colour space depends on a multitude of factors

Files created on a computer screen are created in RGB, either sRGB or Adobe RGB depending on the user's preference. Photographers tend to prefer RAW, which stores the full resolution of the image. Depending on the camera's image sensor this could be twelve or 14-bit data for the maximum detail. This ensures the greatest flexibility for output processing. These are all different colour spaces, each with their own characteristics, which means colours defined in one colour space may not appear the same in another.

There are many reasons why colours appear differently in different viewing environments, on different substrates, or when produced with different print methods. One of the key difficulties is this concept of colour space, the three-dimensional description of all the possible colours that can be mathematically represented in a given space.

Most people understand that RGB and CMYK are respectively additive and subtractive colour spaces. However, it is a mistake to assume that most people understand that there are multiple RGB and CMYK spaces. For example, the CMYK colour range that can be printed on a wide-format printer will not necessarily be the same as that which can be printed on a conventional offset press. The differences between sRGB and Adobe RGB are such that some images may get squeezed and appear differently because sRGB has a smaller colour gamut.

The size of a colour gamut depends on the size of a colour space, which in turn depends on the maths. For instance, CIELa*b* is a massive colour space, one that defines more colours than we can perceive. This is why the ICC converts all colours to CIELa*b* when defining data for a new output colour space. Colour space matters: the bigger the better.