There are many color description and notation formats. Each format represents color in a “theoretical form” that is used to describe useable colors and, to some degree predict how they will interact and can be mixed.
What is color?
Color is a primary response of a human eye to a small range of electromagnetic radiation that is visible to the human eye and is responsible for the sense of sight. Color is a scheme of categorizing and describing this radiation and exists only as perceived by human eyes. There is considerable research about perception and use of color by animals and human sensing of color by other means, however, these subjects are not of interest regarding the artistic use of color. A human eye is an amazingly complex organ whose functioning is still a subject of extensive research by physicist. If you are interested, there are many books and Internet documents regarding the subject.
Basically, color is a characteristic of light energy, which is electromagnetic radiation that has been penetrated a human eye and stimulated receptors in the retina. Electromagnetic radiation is energy that moves at the speed of light as it moves, light energy varies at a very high frequency. Different frequencies have different properties and effects. Compared to the measurable range of electromagnetic radiation, visible color is a very narrow frequency band