A spectroscopic overlay metrology system and corresponding spectroscopic overlay metrology methods are disclosed herein for improving overly measurement accuracy, optimizing overlay recipes, and/or minimizing (or eliminating) asymmetry-induced overly error from overlay measurements. An exemplary method includes generating a diffraction spectrum by an overlay target from incident radiation having more than one wavelength. The diffraction spectrum includes a plurality of positive ordered diffracted beams and a plurality of negative ordered diffracted beams that are separated by wavelength, such that the diffraction spectrum includes more than one wavelength of a positive order and a negative order. The method further includes collecting intensity information associated with the diffraction spectrum generated by the overlay target from the incident radiation, and generating overlay information from the collected intensity information, where the overlay information includes contributions from asymmetry-induced overlay error. In some implementations, the method includes optimizing an overlay recipe based on the generated overlay information.