WISE color−color diagram for the BH12 sample with S/N(W3) > 2 and z ≤ 0.1 (gray dots). Red filled circles represent our BL Lac candidates in the same redshift range. The black tracks indicate the sequence followed by objects where a power-law component F_ν ~ ν^{− α} of increasing strength is added to the emission of an elliptical galaxy, represented by the black dot. This was derived from the SWIRE template of a 13 Gyr elliptical galaxy at z = 0.1. Different tracks correspond to different values of α in the range 0–1.