Delta E (ΔE)

Delta E (ΔE) is the standard metric for color difference. A ΔE under 2 is professional grade (imperceptible to most humans). 2-4 is excellent for daily use. Above 4, trained eyes can see a difference. Above 10, the difference is obvious. Consumer monitors typically achieve ΔE 3-8 out of the box; calibration with a colorimeter brings ΔE under 2. Professional color-critical work (photo editing, print proofing) requires ΔE under 2.