Absorbance spectra and colour for a coloured soda-lime-silica glass can be predicted from colorant concentrations and melting conditions; and conversely: the concentrations for each colorant ion, valence and site (octahedral, tetrahedral) can be "measured" from an absorbance spectrum. The necessary accurate mathematical descriptions for the absorbance spectra of the two most common colorants and contaminants, Cu and Fe, have been developed over the wavenumber range extending from UV to Near IR (200 - 3300 nm) in a soda-lime-silica (NCS) glass, singly or doubly doped with Cu and Fe. The effect of melting conditions on the absorbance spectra are studied over a concentration range from contamination level 0,01 to 2 mol % of added colorant. The obtained fitted spectra have been used to analyse the mutual redox ratios and concentrations of the Fe and Cu ions and sites for a number of contaminated, singly and doubly doped glasses at varied concentration levels, melted in reducing and oxidising conditions. The obtained absorbance spectra are in accordance with the known ligand field theory and each fitted absorbance peak has been identified, and the site configurations for each ion identified. Accurate fitting parameters of the absorbance spectra of Cu and Fe ions have been obtained for summed Gaussian peaks for each valence (Cu"', Cu2`, Fe2+ and Fe 3) and site (Fe2+ octahedral and tetrahedral, Fe 3+ octahedral and tetrahedral) in NCS glasses melted in oxidising and reducing conditions. The fitting parameters (peak heights, i. e. absorbance coefficients in (cm*mol %)-' units, peak positions and peak widths) have been defined and calibrated with known data for all five peaks of Fe2+ ions, nine peaks of Fe 3+ ions, four peaks of Cu2+ ions and one peak for Cu'+ ions, including the UV peaks of all these ions that define the UV-edge of the absorbance spectra and in most cases extend far into the visible range. Equally accurate fitting parameters and functions are developed also for background loss corrections of absorbance caused by surface reflections, OH-bands and IR-edge. For the first time the fitted spectra are shown to scale with Fe and Cu concentrations so well that the developed fitting method can be meaningfully used to analyse doubly and singly doped glasses melted under various conditions. Small, of the order of 0,002 mol % changes in site concentrations, e. g. for Fe 2+ ions in tetrahedral sites, can be repeatedly measured. The melting conditions have a complex effect on the spectrum of Fe 3+ ions in tetrahedral and octahedral sites, changing the proportion of the concentrations. Fe 2+ ions cause significant absorbance over the entire wavelength range studied. Several other novel results are also reported.