Colour Tolerance - how close is close enough?

We all have an opinion about how close two colours have to be to one another to be “acceptable” to meet customers’ expectations. Up until now there has been no good way to quantify that value, but now we have a process to help quantify values associated with acceptability.

This is an edit of an article By David Hunter that appeared in the IPA Bulletin in January/February 2009 - you can read it in it's original form by clicking HERE

People in our industry responsible for producing accurate colour matches are always searching to achieve that perfect colour match for their customers. Of course, some customers are pickier than others and through an often painful trial-and-error process colour producers eventually learn how close of a colour “mis-match” they can get away with. Unfortunately, such a trial-and-error process results in: 1) Losing money on jobs when attempting to satisfy unrealistic customer expectations for many reasons, including operator inconsistency, device inconsistency, and colour judgment inconsistencies. 2) Not knowing what a customer expects until after numerous experiences with trying to attain a colour match. 3) Overselling a customer on a proof that might be more accurate than the customer needs.

Wouldn’t it be nice if we could determine what level of colour match a customer desired before running a job for them? In this way we could: 1) Determine if the measurement and output devices can achieve the desired accuracy. 2) Provide accurate price quotations commensurate with the degree of colour accuracy desired. This would allow for higher-cost proofs versus lower-cost proofs, as well as allowances for re-makes if the customer’s tolerances are on the edge of what our equipment and/or training and/or operator skills are capable of. 3) Ideally, provide a proof of attainment of a certain level of defined accuracy at the completion of a job.

Attempts in the past at determining acceptable tolerance levels have focused mainly on densitometric data, which do not correlate well with how the human eye distinguishes colour difference. For example, a .05 of density difference in a saturated red will look quite different from the same .05 density difference in a light gray. Densitometric data are good for ensuring that one device reproduces colour consistently to itself and are often used in calibration procedures to compensate for device drift. Obviously, we need a different method.

Fortunately, colour scientists have developed two accurate formulas that better correlate how a human sees colour differences. Known as Delta ECMC and Delta E-94, they allow anyone with the proper equipment to quantify colour difference in a uniform manner based on how the human eye sees and interprets colour. The formulas are designed so a unit of measure of “1 delta E” becomes the minimum level a trained human observer can perceive a colour difference while also taking into consideration three important colour attributes—hue, saturation, and lightness. These considerations provide important distinctions because our eyes will notice a shift in lightness or hue before perceiving a shift in colour saturation.

Using the delta E methodology, a difference in 5 delta E units for a light gray will be seen as the same difference in a 5 delta E for a saturated red. So now we possess a standard way to define colour difference that can be defined as a customer’s tolerance.

Now we have a standard to define Tolerance, but how do you know what is right for you and your customers? A unique and proven methodology to define each customer’s Colour Tolerance is to use the Pilot Colour Tolerance Exercise. The exercise consists of a tool whereby multiple colour-squares are precisely manufactured to differ from one another by a certain delta E value. The Pilot Colour Tolerance exercise includes a self-guiding manual to ensure users first evaluate the colours in the correct colour-temperature light and then visually determine which colours match and which do not. This allows users to then determine their colour tolerance expressed by a quantified delta E number value.

Using this approach, it is easy to get a person, group, department or customer defined in terms of their level of Tolerance, thus allowing us to make intelligent decisions that we did not have the ability to make before—without some amount of guessing. Once the customer’s Colour Tolerance has been defined, the colour producers can make the necessary production and business decisions required. This will include determining if our printers are capable of reproducing colour across the imageable area consistent to the level of Tolerance required. In general, to maintain smaller delta E Colour Tolerances greater amounts of time, speed, and monetary investment are to be required. Agreement on this point is essential to a successful and profitable relationship between colour producers and customers.

This is a practical side of colour management, the side most people have not heard about; or if they have heard about it, they do not know how to implement it. For too long, this has been the exclusive domain of the experts. However, a group of industry experts set out to change this by creating the IPA Colour Management Professional (CMP) Certification Program—a comprehensive colour management program designed to provide a consistent worldwide understanding of the best practices associated with colour management implementation in a graphic communication production environment.

Editors Note: Craig Hudson, Managing Director of Hudson Display Services Ltd was the first UK based IPA certified Colour Management Professional and the first Worldwide to achieve "Master" status. Craig highly recommends the CMP Certification Program to anyone involved in the purchase or supply of print. "Discussing colour accuracy has traditionally been a dark art. A discussion of colour might run along the lines of "...a slightly warmer tone... a more reddish orange... a colder blue but deeper..." These terms are useful, and experience develops a descriptive language of colour, but it's entirely non scientific and instinct based. Whilst it would be sad to see the demise of the flowery language, in a commercial world the more precise we can be in our definitions the better. On the subject of the CMP Certification Program - anyone dealing with supplying or purchasing print, will benefit from the course. If I'm speaking to someone who holds the CMP Certification I know they understand the fundamentals of colour reproduction, and I can discuss their expectations in detail. As a result I can produce graphics that meet those expectations in the most efficient way. This results in a better product, a faster service, and in the most cost effective process throughout. For my staff, passing the CMP Fundamentals course reinforces that they are experts in their field. Obviously I don't expect my clients to have taken the course, but my certification gives my clients confidence that they are dealing with someone who really understands digital colour."