The black and white chromaticity classes have been found to be on a par with the other (chromatic) chromaticity classes.
2 In contrast, the gray paper makes a singleton chromaticity class for all observers, as in the previous experiment. These results shed light on the nature of the achromatic component of object colors (for a review, see, e.g., Heggelund,
1974). Although the old concept of a homogeneous achromatic dimension according to which blackness is simply due to the lack of light (e.g., von Helmholtz,
1867) has been abandoned (Volbrecht & Kliegl,
1998), the lightness dimension, varying from black to white, is a pivotal feature of the current theories of color (Wyszecki & Stiles,
1982) and models of color appearance (Fairchild,
2005). If black, gray, and white belonged to the single lightness dimension, then they would make a common chromaticity class. On the contrary, we found that for all our observers the black, white, and gray chromaticity classes do not overlap. Moreover, many Munsell papers were found not to belong to the black, white, and gray chromaticity classes. Hence, these papers do not contain any of the three achromatic components. This contradicts the existence of an achromatic dimension as such. Indeed, the notion of dimension implies that all the colors share this dimension. For example, if width and height are the dimensions of rectangles, then any rectangle has width and height. There is no rectangle without, say, width. However, as follows from our experiments, not all Munsell papers have an achromatic component.