In this study, the interaction processes between a dye (indigo carmine) and two different macromolecular models were studied with the aim to obtain physical-chemistry information about the dyeing of textiles. Two macromolecules, albumin and dextran (DX), were chosen to simulate wool and cotton fibers during the coloration procedure in water. Proton NMR selective and non-selective spin-lattice relaxation rate measurements were used to monitor the strength of the overall complexation behavior of indigo carmine toward albumin or DX. The affinity index, a quantitative parameter related to the strength of the ligand-macromolecule interaction, was determined from selective spin-lattice relaxation rate enhancements due to the bound ligand molar fraction. Moreover, this approach allowed the calculation of the equilibrium constant of the complex formation (K) between the dye and macromolecular models. NMR data suggested a higher indigo carmine-albumin complex thermodynamic stability with respect to the indigo carmine-DX adduct. These results indicate a stronger persistence of the dyeing process in wool with respect to cotton fibers, in agreement with literature data.