Substances with a Single Visible Absorption Band Cannot Be Green

: Some chemistry textbooks use an inappropriate colour model when discussing the colour of a substance that absorbs light at a specific wavelength. More specifically, these textbooks state that substances that absorb only in the violet end of the visible spectrum are green-yellow and substances that absorb only in the red end are blue-green. This paper presents a more accurate approach.

Pierre Funck studied chemistry and physics at ETH Zurich and obtained his PhD in theoretical chemistry in 1995.He works as a lecturer at ETH Zurich and teaches chemistry for bachelor students in the first and second year.
In colour theory there is a long-standing tradition of representing the colour spectrum with a wheel; some examples are shown in Fig. 1.One of the main features of these colour wheels is that complementary colours are opposite to each other.However, the complementary colour of a given colour strongly depends on the colour model (RGB, CMYK, etc.) used.In many of these colour wheels (as can be seen in Fig. 1) the complementary colour of violet is a greenish yellow, and the complementary colour of red is a bluish green.Unfortunately, this has become established in some chemistry textbooks and websites when the colour of a substance is discussed as a function of the absorbed wavelength in the visible spectrum.[6] The purpose of this paper is to provide a more accurate discussion of colour versus absorbed wavelength.
If a substance absorbs only monochromatic light with the wavelength l abs , the observed hue of this substance can be inferred using the CIE chromaticity diagram [7] by drawing a straight line from the absorbed wavelength through the white point, which depends on the colour temperature of the illuminant (Fig. 2).If we use a colour temperature of 5000 K (point D50 in Fig. 2), which roughly corresponds to sunlight, a substance with l abs = 380 nm (the violet end of the visible spectrum) has a hue corresponding to approximately 570 nm (yellow).Another substance with l abs = 700 nm (the red end of the visible spectrum) has a hue corresponding to approximately 495 nm (cyan), which is definitely closer to blue than the colour called 'blue-green'.
A famous example of a substance that absorbs only at the far violet end of the visible spectrum is retinol with five conjugated double bonds (UV-VIS absorption spectrum in Fig. 3).This substance is definitely yellow and not green-yellow. [9]As for the red end of the visible spectrum, the author unfortunately is not aware of any substance that only absorbs at wavelengths around l abs = 700 nm without absorbing shorter wavelengths as well.
As for green substances, it is clear from Fig. 2 that drawing a straight line from, say, 520 nm through the white point leads to Fig. 1.Three examples of colour wheels: the left one [1] and middle one [2] are taken from historical treatises, the right one [3] refers to the RGB colour model.

Received: August 31, 2023
a hue on the so-called purple line that connects the wavelengths 380 nm and 700 nm, and these hues do not correspond to monochromatic light.A famous example for a green substance is chlorophyll, which indeed has two absorption maxima (Fig. 3).
In Fig. 4 the author attempts to make a more accurate rendition of the perceived colour of a substance that absorbs light only in  [8] for inferring the perceived hue from the absorption frequency.
λ/nm 400 500 600 300 all-trans-retinol chlorophyll 700 absorption (arbitrary units) Fig. 3. UV-VIS spectrum of all-trans retinol [9] and chlorophyll. [10]narrow band.The colour spectrum on the upper part (absorption) has been made using the wavelength-to-colour conversion in Wolfram Alpha, [11] and the perceived colour on the lower part using the procedure described above with the white point D50.In conclusion, although the author is perfectly aware that no computer screen -and even less printed paper -is able to reproduce hues of monochromatic light with reasonable accuracy, he recommends using a diagram similar to Fig. 4 when discussing the colour of substances with a single visible absorption band.

Supplementary Material
In order to provide a truer reproduction of the colours under discussion, the original graphics files of Figs 1-4

Fig. 4 .
Fig.4.Hues of substances with single absorption bands in the visible-light spectrum.