Surface brightness of lines of sight through M31's stellar halo as a function of the median metallicity. Values are computed using all M31 halo stars in the field, and points are color-coded by the fraction of M31 halo stars associated with tidal debris features. The large red points show the mean values for fields with and without identified substructure. The dotted line denotes the faintest tidal debris feature identified in our survey, while the dashed line denotes the faintest field observed. Fields with identified substructure are preferentially more metal-rich and higher surface brightness than fields without.