PW relations in different filter combinations [V,(B − V)] (upper left panel), [I,(V − I)] (upper right panel), [K,(V − K)] (lower left panel) and [K,(J − K)] (lower right panel) for the two single-He abundance populations, as labeled. Colors refer to populations with different ages as reported in the upper left panel. In each upper (lower) panel, the solid blue (red) line is the PW relation derived for Cepheids with Y = 0.25 (Y = 0.35), while the dashed black line indicates the PW relation derived by combining the stellar populations with different helium abundances.