Same as Fig. 6 but comparisons with models of two stellar populations: the cases with models D (Pop III) and Ac2 (underlying population with a subsolar metallicity, 1/5 Z_⊙, and a normal IMF) in Table 4. A constant star formation of 100 Myr is assumed for the underlying population. The SMC extinction law is assumed for panel (a) but the Calzetti attenuation law is assumed for (b). The median IGM attenuation is assumed for both the panels. The short-dashed curves are the sequences of the colour as a function of the mass fraction of Pop III stars for dust-free underlying stellar population. The solid, dot–dashed and long-dashed curves are the same sequences but for dusty underlying population with E(B−V) = 0.1, 0.2 and 0.3, respectively. Note that the Pop III stars are assumed to be always dust-free. The positions for the mass fraction of 0.1, 0.01, 0.001 and 0.0001 are indicated by dotted curves with squares, triangles, asterisks and plus-mark, respectively. For Pop III stars, the contribution of escaping nebular LyC is taken into account, assuming the escape fraction of 0.5. The colour sequence with other escape fractions is shown by the triple-dot-dashed curve as in Fig. 12. For the underlying population, the escape fraction of 0.01 is assumed.