Three-component thermochromic mixtures generally consist of a leuco dye, a phenolic colour developer, and a high-melting organic solvent (listed in order of increasing mass). Their colouring behaviour is controlled by competing binary interactions, with the dye–developer interaction responsible for colour formation and the developer–solvent interaction responsible for solid-state colour erasure. In the present study, the reversible thermochromic properties of twelve systems of three-component mixtures comprising crystal violet lactone (CVL, dye), one of four alkyl gallates (developer), and one of three long-chain alkyl alcohols (solvent) were examined, as bulk samples, with slow heating and cooling rates. The thermochromic behaviour (i.e., melt-lightened vs melt-darkened thermochromism) of these mixtures was examined as a function of the difference in length of the alkyl chains attached to the developer and solvent. When the alkyl chain lengths were well-matched, the developer–solvent interaction dominated in the solid state and melt-darkened thermochromism was observed. When the alkyl chain lengths were poorly matched, the dye–developer interaction dominated in the solid state, and melt-lightened thermochromism was observed. Ternary thermochromic phase diagrams define compositional regions of optimal thermochromic behaviour (i.e., demonstrating high colour contrast), showing the intricate balance between developer–dye and developer–solvent interactions, and component concentration, in determining thermochromic behaviour.