Color QC and Matching Blog


Posted by Mike Burns on Thu, Jul 16, 2009

One of the more frequent questions we hear from suppliers is:  What should my color tolerances be?  The simple answer equates to what the client will accept in color variation in all directions of color space versus the color standard the supplier needs to match.  The importance of the client providing representative color standards will be the subject of a future blog.

In the seventies and throughout the eighties the majority of suppliers relied on the CIELab defaults of +/- 0.5 for *a / *b axis with *L tolerances of +/- 1.0.  The CIELab DeltaE default was 1.0.  These so-called box tolerances were applied to dark colors, light colors, pastel colors, or high chroma colors with the assumption that regardless of where color was in color space any change in color perceptibility or color acceptability for the color match was the same.  The issues with these default numerical tolerances were that the human eye could accept bigger differences in lightness and darkness of darker colors than in lighter colors.  The same could be said of the hue (color of the sample).  Light pastel colors were more susceptible to a noticeable color difference than darker colors.  Consider a light beige where the color shifts a numerical 0.5 towards the -b* axis (bluer).  Dependent on the product this shift may be unacceptable whereas in a dark brown or maroon color it may be totally acceptable.  There have been shifts of Delta L* (Lightness) of 3.0+ units in a dark royal blue that have been totally acceptable whereas the Delta L* tolerance of 1.0 would indicate it was unacceptable.  With these type analogies color scientists sought some way to apply weighting factors to better define color differences dependent on where the color was located in color space.

In the 1980's and 90's, color scientists developed improved color equations like CMC and later CIE2000 for determining a better way to define small color differences that were more perceptually uniform.  Both equations use elliptical tolerancing to define an acceptability ellipse for a point in color space.  The ellipses vary in shape and size throughout color space.  Both CMC and CIE 2000 DE use weighting factors for lightness, chroma (CMC) and lightness, chroma, and hue (CIE2000) that better correlate to color differences asa the human eye sees it.   The result over time has been that elliptical tolerances as defined by color difference equations like CMC and CIE2000 are widely preferred within the color industry as being superior in defining human color perceptibility and color acceptability.  However, box tolerances are still in widespread use due to their ease of use and the long and extensive color histories that companies have bases on them.

 There is no right or wrong answer.  It's up to the buyer and seller to negotiate what is acceptable and to clearly spell it out in terms of a color specification.

Tags: color tolerances, Announcements, Color difference, Product Information, box tolerances, CMC DE, DE CIE2000, elliptical tolerances, color standard