I'm guessing the QC manager is new to our industry. Delta-E (dE) calculations result in a single number that represents the "distance" in Lab color space between two colors--the larger the number the greater the difference. It has become an increasingly critical metric. I'm assuming that your QC person is using dE2000, as this is the most appropriate calculation to use. Most companies check dE by reading color bars, often in a closed-loop system. The acceptable tolerance is established by the printer (often tied to specifications) and, for some jobs, in conjunction with the customer. Readings are generally taken in solids, flat tints, or spot colors and not normally in the image area (and only then if it is a critical brand color). By only measuring a few randomly selected patches, possibly colors in which a slight difference would not even be noticeable to the client, you risk rejecting jobs that the client would have found acceptable. Your requirement of 3 dE or less is tighter than GRACoL specifications. Encourage your QC manager to take our online Color Management Fundamentals course.