Most young, ultracool substellar objects with spectral types later than M9 show very red mid-infrared colors up to 24~micron, significantly redder than expected for their optical and near-infrared spectral classifications. These objects have estimated ages and masses in the intervals 20-300 Myr and 12-35 times the mass of Jupiter, respectively. According to optical data, their atmospheres have low gravity and are rather cool with characteristic effective temperatures between 1300 and 2400 K, rather close to the temperatures of close-in giant planets around solar-type stars. We focus on the particular case of G196-3B, an L3-type substellar companion orbiting a young low-mass star. We discuss various physical scenarios to account for its reddish nature and conclude that a low-gravity atmosphere with enshrouded upper atmospheric layers and/or a warm dusty disk/envelope provides the most likely explanations.