Complexities on the roles of reference color gray and grayness are reviewed. They are essential in color appearance, but gray is an implicit color. Although grayness is not explicitly used in visual color assessment of surface colors or color order systems, gray can be combined with any colors having six primary-color components using the term grayish, for example, grayish red and grayish yellow. However, the existing region of grayness is limited in a part of color-appearance space. Illuminance dependency of gray perception is also clarified. Existence of two kinds of psychometric quantities are suggested: one is the attribute of grayness based on its psychological amount in a grayish color under study, and the other is the attribute of brightness of the grayish color under a specified illuminance, psychophysical quantity. The Nayatani-Theoretical color order system, which uses three opponent-colors axes, can clarify the above complexities of gray and grayness. Its importance is the same as six primary colors, red-green, yellow-blue, and white-black. (c) 2012 Wiley Periodicals, Inc. Col Res Appl, 39, 37-44, 2014