The elements of colour II: the attributes of perceived colour

Abstract

In most educational contexts, colour is presented as having a single set of three attributes or dimensions. Just three attributes suffice to describe colour as long as we consider only a single mode of colour appearance, such as colours perceived as belonging to light-reflecting objects, where the CIE-defined attributes of hue, lightness and chroma are sufficient, as are the three attributes used in the Natural Colour System (NCS), hue, blackness and chromaticness. But other attributes come into play when we consider colours perceived as belonging to light itself, including (1) light perceived to be falling on objects, or (2) light reaching the eye, whether directly from a primary light source, or by specular reflection, diffuse reflection or transmission by objects. More than three attributes are therefore required to fully describe the appearance of illuminated objects, which involves colours in multiple modes of colour appearance. This paper provides a discussion of the main modes of colour appearance followed by illustrated explanations of the six attributes of perceived colour currently defined in the CIE International Lighting Vocabulary, hue, brightness, lightness, colourfulness, chroma and saturation, along with the NCS-defined attribute of blackness and the related attribute of brilliance. Special consideration is given to the distinctions between brightness and lightness, and between colourfulness, saturation and chroma, and to the relevance of these concepts for understanding, describing and depicting the appearance of illuminated objects. Also of special interest is the influence of chromatic intensity on brightness and lightness perception (the Helmholtz-Kohlrausch effect), which I argue is connected with blackness perception, and which must be contended with in order to determine lightness in the Munsell system and CIE L*a*b*. A second issue relating to colour attributes in colour education is that, with some exceptions, hue is usually presented using just a single hue circle or “colour wheel”, very often in a form embodying historical beliefs about three “primary colours”. To address this issue, the section on hue discusses different kinds of simple hue circle that emphasise different relationships among hues and provide alternative hue frameworks.