The optical absorption bands of an electrochromic mixture of oxides can be adjusted systematically by varying the ratio of constituent oxides. The oxides in this study were CoO, FeO MoO3, NiO and WO3. The way optical absorption maxima (at constant insertion coefficient x) vary with mole fraction of constituent oxides is best predicted in terms of the optical-shift parameter S via the frequency maximum of the optical band, in a manner akin to optical basicity boolean AND. S is followed by bands originating from optical transitions on specific ions and by bands due to charge-transfer and intervalence transitions between ions. It is also shown that computer speciation modelling is a good predictor of the compositions of metal-oxide mixtures prepared by electrodeposition from aqueous solutions.