What is COLOR? 

There are several definitions, but one of them seems very practical for the systematization of knowledge.

Color is a function of three components:

• Object
• Observer
• Lighting/Illuminant

Let us define the components more precisely

Object

All objects when illuminated may absorb different wavelengths differently. This is the component that we can describe more or less precisely.  The problem is that color is only one component - the visual appearance of the sample is related to many more additional parameters like surface, glossiness, and much more...

Observer

Everybody is different. We are perceiving the surrounding world differently. No two of us see the same. We can statistically try to create an average "observer" and we have created "Standard Observer" published as ISO standard, but it is very individual and there is no fully objective way to measure and numerically describe personal differences... 

Lighting/Illuminant

Light is a visible electromagnetic wave that may have a different wavelength/frequency. Most of us are capable to see light from the range of 400 -700nm. Below 400 we have UV above IR.  

What we know the best is daylight - a mix of different weaves that the star Sun is emitting. Daylight fluctuates throughout the day, seasons, and varies depending on your location on Earth.

We also know the light that is produced by other sources like candela, fire, classic tungsten bulbs, fluorescent tubes,s or LEDs. 

Standards

Standard Illuminant

A standard illuminant is a theoretical source of visible light defined by an SPD (Spectral Power Distribution) curve which is published. The International Commission on Illumination  (CIE) is the international body responsible for publishing all of the well-known standard illuminants. 

Standard Observer

Humans perceive color and appearance subjectively and differently, even with a trained eye. The standard observer as a numerical representation of what the “average person” sees. In 1931 the CIE published the 2° Standard Observer based on their research. This Standard Observer is called 2° because, during the color-matching experiment, subjects looked through a hole that allowed them a 2° field of view. 

In the 1960s, it was realized that the cones in the human visual system cover a larger field of vision than previously thought. This led to another definition of the standard observer - this time based on 10°

Standard Observer and Standard Illuminant are two theoretical models that are required for mathematical calculations. You have to remember that these models were created a long time ago and their precision is not perfect. It is also important that these are averaged models - and especially in the case of observation, we know that individual features can be quite distant from the average value. Thus, these are mathematical models that may not coincide with the individual's perception of color.

Color phenomena

In order to understand the relativity of color, it is important to understand the phenomena that affect the perception of color in a non-obvious way by both man and instruments.

Fluorescence

Fluorescence is the process where a material absorbs light at high energy, short wavelength, and emits light at lower energy, usually visible, wavelength. To understand the practical significance of this phenomenon, we need to know that modern materials science uses substances called OBA (Optical Brightener Agent) - their purpose is to give the appearance of pure white in materials such as paper - which are objectively not as white as they appear when lit by light with the participation of UV.

UV radiation, invisible to the eye, is converted by OBAs into blue light - artificially boosting some blue wavelengths. The effect of such action is such that for certain wavelengths the amount of reflected light exceeds even 100%. Depending on how much invisible UV component there is in the light illuminating the object, its color changes, giving the impression of luminous white in the case of paper. Similar substances are used in the production of fabrics, paints, and plastics. Their use is quite common but difficult to assess visually.

Metamerism