Colloidal lead chalcogenide (IV-VI) quantum dots and rods are of widespread scientific and technological interest, owing to their size tunable energy band gap at the near-infrared optical regime. This article reviews the development and investigation of IV-VI derivatives, consisting of a core (dot or rod) coated with an epitaxial shell, when either the core or the shell (or both) has an alloy composition, so the entire structure has the chemical formula PbSexS1-x/PbSeyS1-y (0 ≤ x(y) ≤ 1). The article describes synthesis procedures and an examination of the structures' chemical and temperature stability. The investigation of the optical properties revealed information about the quantum yield, radiative lifetime, emission's Stokes shift and electron-phonon interaction, on the variation of composition, core-to-shell division, temperature and environment. The study reflected the unique properties of core-shell heterostructures, offering fine electronic tuning (at a fixed size) by changing their architecture. The optical observations are supported by the electronic band structure theoretical model. The challenges related to potential applications of the colloidal lead chalcogenide quantum dots and rods are also briefly addressed in the article.