We propose a mathematical model to derive the chromatic parameters from increment spectral sensitivity functions. This model was applied to determine the effective red, green, blue, and yellow mechanism contribution to the detection of the spectral stimuli of five normal trichromatic subjects. Detection thresholds were measured for a $300\mathrm{ms}$ , 1.2° circular test flash presented on a $100\mathrm{cd}∕{\mathrm{m}}^2$ white background for spectral wavelengths between 410 and $660\mathrm{nm}$ . The model analysis confirmed that in the red–green wavelength area, the detection of our chosen stimuli was mediated by two distinct (L–M) antagonistic mechanisms: a red–green and a yellow, from the blue–yellow system. We inferred that the red–green mechanism receptive fields consisted of a single L- or M-cone center with a homogeneous or heterogeneous surround devoid of S-cone projections. For the receptive fields of the yellow half of the blue–yellow mechanism, we propose a similar configuration but with S-cone projections present in the surround. This proposal is not concordant with what is currently understood regarding retinal physiology. However, two L–M antagonistic mechanisms in the red–green wavelengths as proposed by our results predict what would appear as an intuitive yellow mechanism with a maximal sensitivity at the $578\mathrm{nm}$ wavelength, where the red–green mechanism sensitivity is null.