Infrared colours of subdwarfs as a function of spectral types (increasing spectral types to the right, where 0 ≡ M0 and 10 ≡ L0). We plot new and known subdwarfs, extreme subdwarfs, and ultrasubdwarfs as red squares, green circles, and blue triangles respectively. Small black diamonds are solar-metallicity M dwarfs from the DwarfArchives.org website with their AllWISE photometry, as well as M and L dwarfs from Kirkpatrick et al. (2011) with their WISE photometry. We highlighted as cyan stars field L dwarfs with features of L subdwarfs (Sect. 7).