The aim of the study was to understand the thermal reactions of pyrotechnic flash compositions comprising ternary mixtures of potassium nitrate, Aluminium and Sulphur, and attempted to optimize the stoichiometry of the composition through residue analyses for best results. Differential Scanning Calorimetry (DSC) was used for the thermal reaction studies, and X-ray diffraction (XRD) and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS) for the residue analysis. The flash compositions containing KNO3/Al/S were more reactive under confined test conditions (sealed container) than in unconfined conditions (open container). The residue analysis results showed that the optimal stoichiometric mixture for pyrotechnic flash composition (KNO3/Al/S) was 53/33/14 by weight. The stoichiometric composition was experimentally validated by proving complete combustion and this mixture was found to have the highest heat release (637 J/g) of all the compositions studied.