The human eye, and therefore human perception, is sensitive to the range of light wavelengths that constitute the visible spectrum, including the corresponding spectral colours from red to violet.
Light, however, is rarely of a single wavelength, so when an observer notices a red ball they are probably seeing a range of similar wavelengths of light within the visual spectrum.
Perception of colour is a subjective process as our eyes respond to stimuli produced by incoming light but each of us responds differently.
The environment in which colours are observed and the type of object can influence colour perception.
The physical properties of light, including wavelength and intensity, can affect how colours are perceived.
An observer’s physical and mental state can affect their perception of colour. Health, medications, mood, emotions, or fatigue can all affect an observer’s eyes, vision, and perception.
Different observers may perceive colours differently based on their life experiences, linguistic backgrounds, and educational, social, and cultural factors.
The term “observer” has distinct meanings in various scientific fields, such as special relativity, general relativity, quantum mechanics, thermodynamics, and information theory.