The brown dwarf and imaged exoplanet color–magnitude diagram. 51 Eri b is shown with the red star. The colors of 51 Eri b place it among late-T dwarfs, where it is redder than most comparable-temperature brown dwarfs, likely indicative of greater cloud opacity in the atmosphere. The photometry for the field M-dwarfs (black circles), young M-dwarfs (blue triangles), field L-dwarfs (dark gray circles), young L-dwarfs (light blue triangles), and T-dwarfs (light gray circles) is taken from the compilation of Dupuy & Liu (2012) and Liu et al. (2016). We used a linear fit to convert WISE W1 photometry to LP, similar to what was done in Macintosh et al. (2015). The photometry for the directly imaged planets and young brown dwarfs were taken from Males et al. (2014), Bonnefoy et al. (2014), Bowler et al. (2017), Marois et al. (2010), Chauvin et al. (2005), Rameau et al. (2013), Naud et al. (2014), Leggett et al. (2007), Delorme et al. (2017), Goldman et al. (2010), Janson et al. (2011), and Kuzuhara et al. (2013).