Double-metallic lead-free halide perovskites, Cs₂B^IB^IIIX₆, sharing three-dimensional crystal structure, have been under the spotlight as the promising alternatives for the toxic and instable lead-based counterparts. Interest in Cs₂B^IB^IIIX₆ motivates intense research into their colloidal nanocrystals (NCs). Recently, Cs₂B^IB^IIIX₆ NCs have made great progress in the optical performance via alloying or doping, but there are still great challenges for optoelectronic applications. In this review, the latest advances of Cs₂B^IB^IIIX₆ NCs in synthesis approaches, bandgap engineering, photoluminescence (PL) optimization, and applications are summarized. The focus is put upon the composition–property relationships of Cs₂B^IB^IIIX₆ NCs, which is approached by discussing the influences of composition variation on the electronic states, carrier dynamics, and optical properties. The challenges and the corresponding improving strategies in the development of high-effective and stable Cs₂B^IB^IIIX₆ NCs for device applications are also highlighted. It is believed that this review can deepen the understanding on this burgeoning material system and shed light on their future research directions.