What is the minimum ΔE that a human eye can detect?
This is not at all a simple question with a single answer. As you note, the answer depends on the ΔE method or equation used, but it also depends on many other factors such as the viewing conditions for the samples (size, color, separation, luminance level, gloss, texture, background, etc.). As a very general rule of thumb, people take a simple CIELAB ΔE*ab of 1.0 to approximate a just noticeable difference. However, the CIELAB space is known to be non-uniform for color differences and it might take a significantly larger ΔE*ab for the difference to be noticed for a high chroma color. This non-uniformity is what prompted the development of weighted color difference formulas such as CIE94, CMC, and CIE2000. Since those weightings tend to reduce the average color differences, a rule of thumb for just noticeable color differences tends to be a bit smaller, closer to 0.5, when using those equations. In all cases, these are very general rules and shouldn’t be taken too seriously. It is alwa
