Comparison of the behaviour of colour as a function of age and metallicity for the E-MILES, M09, C18, and XSL SSP models (see legend). Left panels: colour as a function of age. Right panels: colour as a function of metallicity. Top row: (B − V). Top-middle row: (V − I). Bottom-middle row: (I − J). Bottom row: (J − K). Ages span from 1 Gyr to 16 Gyr, and metallicities span from [Fe/H] = −0.4 dex to [Fe/H] = +0.2 dex (left panels); [Fe/H] = −2.2 dex (XSL PARSEC/COLIBRI) or [Fe/H] = −1.7 dex (other) to [Fe/H] = +0.2 dex (right panels). We note that E-MILES models are not safe to use in the NIR below [Fe/H] = −0.4 dex but are included for illustrative purposes. Solar-metallicity E-MILES models are shown in heavier line strengths than the sub- and super-solar models of the left panels. XSL SSP sub- and super-solar models are represented by shaded areas, centred on the solar metallicity. We note the different colour-scale values between the same colour panels.