The thermal history of Co²⁺-doped hopeite Zn₃(PO₄)₂·4H₂O was investigated combining thermogravimetric analyses, X-ray diffraction and UV-Visible spectroscopy. For the two dehydration steps, Zn₃(PO₄)₂·4H₂O → Zn₃(PO₄)₂·2H₂O → α + δ-Zn₃(PO₄)₂, the temperatures increases linearly with the Co²⁺ doping rate (130–340 °C). The coexistence of α and δ phases formed after dehydration was explained considering a complex and unusual intergrowth phenomenon. Finally these phases transform into the high temperature stable phase, γ-Zn₃(PO₄)₂, at a temperature also controlled via the Co²⁺ doping rate (350–900 °C). Both phase transformations are associated with variations of Zn²⁺/Co²⁺ local environments, from a distorted octahedral site in hopeite to a distorted tetrahedral site in Zn₃(PO₄)₂·2H₂O or α + δ-Zn₃(PO₄)₂ and finally to distorted octahedral/trigonal bipyramid sites in γ-Zn₃(PO₄)₂. The successive changes of the Co²⁺ coordination induce colour variation from pink (octahedral coordination) to blue (tetrahedral coordination), then violet (5 and 6-fold coordination). Co²⁺-doped hopeites can so be used as efficient over-heat temperature indicators.