The general concept of colour for the scientists is related to the characterization of a spectral quantity, as a result of the visible electromagnetic radiation interaction with matter (chemical compounds). Consequently, colour is not a characteristic inherent property of materials since it depends on their selectivity to absorb or reflect the incident radiation at certain frequencies of the visible spectrum. Thus, the colour stimulus depends on the nature, chemical composition and structure of a material via a unique way. Moreover, it is characterized by its spectral reflection or absorption curve (frequency spectrum of a certain intensity), within the visible spectrum limits. In particular, the spectrum is determined via the reflection ( \(R\%\) ) or absorption ( \(A\%\) ) coefficients at wavelengths \(\lambda \) from 380 to 780 nm as depicted in Fig. 1.