Period-amplitude diagrams for Cepheids in multiple bands. The filled circles and squares represent FU and FO Cepheids, respectively. In the case of G band, the theoretically predicted amplitudes for metallicities (Z = 0.02) representative of Cepheids in the MW are also shown for different masses (De Somma et al. 2022). The solid and dashed lines represent FU Cepheid models, while asterisks show FO models. In grizJHK_s bands, overplotted Cepheids in open star symbols are discussed in Sect. 3.3.