We present a first theoretical evaluation of a new optical technique for diffraction grating metrology. While well-known spectroscopic ellipsometry (SE) is based on classical ellipsometric spectra taken in the usual planar diffraction geometry, we propose to use spectrally resolved full Mueller matrices of the gratings measured in the most general geometry of conical diffraction. We simulate the case of two superimposed one-dimensional (1D) gratings with an overlay defect, i.e. a relative shift of the two gratings with respect to each other. We show that the proposed new technique is sensitive to both the magnitude and sign of the shift, and thus it should be more efficient than usual SE for overlay characterization in real cases.