A Technical Colour Guide for Printing

In the world of printing, accurate colour reproduction is one of, if not the most important issues. The ability to reproduce colours precisely is critical for companies wishing to have consistent branding.. Achieving the right colours in print involves understanding colour basics, managing colour with precision, and employing the right tools and techniques. This article provides a comprehensive technical colour guide, delving into essential components like colour gamuts, colour matching systems, and the role of spectrophotometers in achieving optimal colour reproduction.

Understanding Colour Basics

RGB vs. CMYK Colour Models

In the world of colour, two primary models dominate—RGB (Red, Green, Blue) and CMYK (Cyan, Magenta, Yellow, Key(Black)). RGB is primarily used for digital displays, while CMYK is the standard for print. 

Understanding the differences and best use cases for each model is important for achieving accurate colour representation in the intended use.

Understanding Colour Gamut and Limitations

The colour gamut represents the entire range of colours that a device or colour space can reproduce. Different devices and colour models possess distinct ranges in available colours. For Instance RGB has over 16,000,000 colours and shades, whereas CMYK only contains around 16,000 colours.
You Can find much more in depth detail CMYK and RGB in our article titled All about colours...in regards to printing

 

Colour Management Systems

What is a colour management system?

A colour management system (CMS) is a set of software, hardware, and processes, designed to create consistent and accurate representation of colours.

Colour Profiles

These are mathematical descriptions of a printing press’ colour behaviour. They define how a printing press reproduces colours, allowing the CMS to compensate for the unique colour characteristics of each press. Profiles are also used for different materials the press prints onto. Further ensuring that colours will always match.

Colour Calibration

Instruments like colorimeters and spectrophotometers are the most accurate and efficient tools used to calibrate and profile printers. They allow printers to create colour profiles, therefore calibrating devices to ensure accurate colour representation and consistency throughout the printing process.

Extending Colour Gamuts with Additional Inks

Additional inks (Green, Orange, and Violet) beyond the standard CMYK, can be added to a printing press to extend the colour gamut. These inks enhance the range of colours that can be reproduced, allowing for more vibrant and accurate colour representation. They are only really useful when using spot colours, as CMYK colours will always print using CMYK.

Advantages and Challenges of using Additional Inks

While extending colour gamuts offers benefits (as mentioned above), it also presents challenges like increased complexity and cost. 

The complexity usually comes from the creation of the spot colours, and the effort needed to print samples, then visually and digitally (using a spectrophotometer) compare how the printed colour compares to the desired colour. Often once the colour has been set up, it is easy as printing presses can usually run automatic calibrations to ensure continued colour accuracy.

The additional cost is down to both the cost of the additional ink used, as well as a reduced printing speed, and the extra maintenance required.

Spectrophotometers in Colour Management

Spectrophotometers are sophisticated instruments used for precise colour measurement and analysis. They can quantify colour by measuring the intensity of a light beam at different wavelengths. These measurements are then used to create colour profiles, as well as compare colours using a calculation called Delta E.

Colour Comparison Using Delta E

In the realm of colour management and colour accuracy, Delta E (ΔE) is the fundamental metric used to quantify the difference between two colours. 

It provides a numerical value that represents the perceptual difference or distance between the desired colour and the printed/ simulated sample colour.

Understanding Delta E (ΔE)

The International Commission on Illumination (CIE) calls their distance metric ΔE
Delta E is a mathematical formula that calculates the Euclidean distance between two points in a three-dimensional colour space. The latest and most accurate version of the Delta E formula is CIEDE2000. This is used worldwide, with the resulting Delta E value quantifying the colour difference based on factors like lightness, chroma, and hue.

Delta E does not use the RGB or CMYK colour models. Instead it uses the LAB or HSL colour model. The key difference between these and RGB is that LAB/HSL covers the entire visible light spectrum.

Interpretation of Delta E Values

The Delta E value provides insights into the perceptual difference between two colours. Here's a general guide for interpreting Delta E values:

• Delta E < 1.0: Imperceptible difference. Colours are essentially identical to the human eye.
• 1.0 ≤ Delta E < 2.0: Slight difference. Some individuals may notice a subtle variation in colour.
• 2.0 ≤ Delta E < 3.0: Noticeable difference. Most observers can discern a difference in colour.
• Delta E ≥ 3.0: Significant difference. Colours are visibly distinct and may not meet colour accuracy requirements.

Limitations of Delta E

• Delta E does not fully account for human vision variations, especially for certain colours and lighting conditions.
• It doesn't consider colour preference or the context in which the colours are viewed.
• The same Delta E value can represent perceptually different differences depending on the colour and region within the colour space.

Pantone: A Colour Matching System

Pantone is the world-renowned colour matching system widely used in the printing and design industry. 

It provides a standardised way of identifying and communicating colours. Ensuring consistency across various mediums and materials.

The Pantone Matching System (PMS) uses a unique numbering system, and a set of tools such as colour book/swatches. These are used by designs and brands when creating brands, then by printing companies such as Sticker it to achieve consistent colour across all methods and mediums of communication.

PMS is not without its issues. Many colours in their colour books, and far from achievable using standard CMYK printing methods. 

Even when including an extended ink set, ~5% of colours remain unachievable. The only way to achieve this is using older non-digital printing technologies such as litho printing. The problem with these older non-digital methods is that often, to be cost-effective, you need to print on very large volumes.