What is Color ?

Colour is a property which can be defined in the sense of physics or in the sense of human neurophysiology. The way humans perceive colors is complicated, but scientifically well understood.

Color of a surface in the sense of physics is the spectrum of light frequencies emitted from a surface. Neglecting active light sources, it is a reaction on the frequencies of the incoming light, and therefore depends on it. Out of physiological reasons, standard colors are defined with respect to a standard light source that represents an ideal daylight. Such devices are used in labs. The standard daylight source has a continuous spectrum of a known shape.

The human eye registers, i.e., projects out of the reflected spectrum three sub-spectra, one centering at red, one at green, and one at blue. The combination of the intensities of these three components produces our sensory impressions of all colors, including violet and yellow. These effects can be reproduced by adequate mechanical sensors and processing.

The surface itself may have unlimited details, microscopic differences in reflection. The reflected light is always the sum of the light reflected from some surface of a certain size. Therefore color is only defined with respect to a certain spot size. Seeing a surface from a certain distance increases the spot size, and hence may produce new integral colors, as well known from a TV screen surface. Also quick change of light ( >25 cps) sums up to an integral sensory reaction.

In addition, our brain has two extraordinary processing functions, which keep our color impression much more constant than physics would make us expect:

A)     White balance: Standard daylight is “white”. Our brain detects deviations of the light source from being white, and corrects the sensory impression very widely in order to produce correct colors as if the source were white. Digital cameras try more or less successfully to simulate this process as “automatic white balance”.

B)     Surface homogeneity: Our brain mysteriously detects shadows and colored reflections on homogeneously colored surfaces and presents itself the corrected surface color. So we recognize for instance a pullover to be uniformly red within complex light conditions, reducing the surface color to the woolen thread color.

Consequently, describing “the colour” of an object is not a question of a simple vocabulary. Even given under standard light conditions, it still needs the definition of the spot size of integration, and the representative surface parts of features meant by this categorization. 

Art conservators recently tend to scan art objects by much more colors than the three colored subspectra the human eye distinguishes. This allows for much better print representation and scientific analysis of colorants.