Seven simulated absorption spectra that span the visible spectrum, are used to probe the degree of linear correlation that exists between real absorbance, Ao, at λmax, and three well-established colour-based parameters, based on the standard Red, Green and Blue scale, sRGB, namely: (i) apparent absorbance, A(sRGB), (ii) apparent fraction of absorbed light, 1-T(sRGB), where T is the apparent transmittance and (iii) colour difference, ΔE. In all cases the colour-based parameter, A(sRGB), linearly correlates best with Ao. This predicted correlation is tested using three different, actual colour-based indicators, using UV/Vis absorption spectroscopy to monitor the change in actual absorbance of each of the indicators and digital photography to monitor simultaneously the change in the values of sRGB, and so A(sRGB). The three different indicators used were: a CO2 indicator, a photocatalytic activity indicator and an oxygen indicator. In all three cases the apparent absorbance parameter, A(sR), derived from sRGB analysis of the digital images, is proportional to the real absorbance, as measured using UV/Vis spectrophotometry, and able to yield the same key analytical information. The increasing use of sRGB analysis of digital photographic images, i.e. digital colour colourimetry, DCC, is discussed briefly.