Colour effects of co-pigmentation of anthocyanin revisited—3. A further description using CIELAB differences and assessment of matched colours using the CMC model

Extensive colour difference calculations (CIELAB and CMC models for CIE D65/10°, Ill./Obs.) were applied to the colorimetric coordinates previously gathered on 224 pure or co-pigmented (by rutin) solutions of cyanin at pH between 2.5 and 5.5. CIELAB coordinates additionally recorded on acidic (0.1N HCl) methanolic and aqueous cyanin solutions (10−5 M−2.5×10−3 M) showed the considerable hue gamut covered by each series of solutions (1/6 and 1/8 of the colour circle). A markedly yellowing effect of water vs. methanol was observable and it represented the most influential parameter in the impressive CIELAB differences (ΔE* up to 38 units) caused by the solvent. On aqueous cyanin solutions, successive one unit pH elevations resulted in huge colour gaps (up to ΔE* > 60 units, at pH 3.5 vs. 2.5). The corresponding ‘loss of colour’ was mainly attributable to a decreasing chroma (ΔC*) while the coupled hue differences (ΔH*) corresponded to more or less deep blueing effects at pH elevations up to 4.5 and more unexpectedly to yellowing effects at pH increased to 5.5. Using CMC difference calculations, the colours of 0.1 N HCl aqueous solutions at low concentrations were best matched by those of cyanin solutions at pH 2.5 and about 2–2.5 fold higher concentrations; at high concentrations, their colours were preferentially simulated by solutions at each corresponding—or lower—cyanin concentration but co-pigmented by rutin (up to 4:1 ratio). Similar, but more complex features, emerged in the constitution of many best matched couples of colours from the database of model solutions at pHs 2.5–5.5. Close—or even identical—colour stimuli were also shown to be based on either closely matched or quite distinctly shaped spectral curves. The complete ‘colour by numbers’ approach presented in this series of papers may find applications in many fields in which anthocyanic pigments are employed, as natural food colorants for instance.