To investigate how rod signals influence hue perception and how this influence can be incorporated into opponent-color models, we measured the shift of unique-hue loci under dark-adapted conditions compared with cone-plateau conditions. Rod signals produced shifts of all spectral unique hues (blue, green, yellow) but in a pattern that was inconsistent with simple additive combinations of rod and cone inputs in opponent-color models. The shifts are consistent with non-linear models in which rod influence requires non-zero cone signals. Cone-signal strength may modulate or gate rod influence, or rod signals may change the gain of cone pathways.