HRDs for metallicities Z = 0.014 (Solar, left), Z = 0.006 (LMC, middle), and Z = 0.002 (SMC, right). Solid black lines show predicted IS boundaries from A16 for the second+third crossings averaged together; dashed black lines show the first crossing. Solid and dashed cyan lines illustrate predicted boundaries from De Somma et al. (2020, 2022, with their case A referred to as a ‘canonical’ mixing-length relation. We note the difference in Z values: 0.020, 0.008, 0.004), for α_ML = 1.7 and 1.5, respectively. Solid and dashed pink lines show predicted sets A (simple convective model) and D (added radiative cooling, turbulent pressure and flux) from Deka et al. (2024), which estimate the envelopes of instability strips across a large range of metallicities ([M/H] ∈ {0.00, −0.34, −0.75}, namely, Z∈ {0.013, 0.006, 0.002}) and are shown in all panels. Observational estimates of 265 fundamental-mode Galactic classical Cepheids from Groenewegen (2020a) are shown in the left panel. Empirically derived IS boundaries for the LMC from Espinoza-Arancibia et al. (2024) are shown as grey regions in the middle panel.