This paper describes a study performed to understand preferences and tolerances for “gray” in reflection prints. A psychophysical experiment was conducted, with observers looking at printed targets containing near-neutral patches in different spatial layouts and under different illuminations. The task was to choose the patch or patches that were perceived as being closest to gray. Spectral measurements were made of these patches, and converted to CIELAB coordinates for each of the chosen illuminations. The observer responses were tallied into preference histograms as a function of the (a*, b*) coordinates of the patches. Results show that regardless of layout or viewing illumination, the peaks of the histograms occurred in the quadrant where both a* and b* are negative. Follow-on experiments were conducted using more complex targets under less controlled viewing conditions. The same trends were observed, indicating that people generally prefer “cooler” grays, and have low tolerance to errors that result in positive values of a* or b*. These observations can be used to derive robust gray-balancing for color output devices.
Raja Bala, "What is the Chrominance of “Gray”?" in Proc. IS&T 9th Color and Imaging Conf., 2001, pp 102 - 107, https://doi.org/10.2352/CIC.2001.9.1.art00019