a) Dependence of the W1 − W2 colour on the oxygen abundance 12+logO/H for a sample of  ~1000 galaxies. Galaxies with Hβ luminosity L(Hβ)  ≥  3 × 10⁴⁰ erg s^-1, corresponding to SFR(Hβ)  ≥  0.7 M_⊙ yr^-1, are shown by red filled circles while galaxies with L(Hβ)  <  3 × 10⁴⁰ erg s^-1 are shown by blue open circles. The NW and SE components of the low-metallicity BCD I Zw 18 are labelled and connected with a solid line. Newly identified galaxies with W1 − W2  >  2 mag are shown by large red filled circles, the three galaxies discussed by Griffith et al. (2011) are shown by large red filled squares. Typical error bars are shown in the lower right corner. b) Histograms of W1 − W2 distributions for galaxies with L(Hβ)  ≥  3 × 10⁴⁰ erg s^-1 (red dotted line) and with L(Hβ)  <  3 × 10⁴⁰ erg s^-1 (blue solid line). c) and d) same as a) and b), respectively, but the sample is split into galaxies with EW(Hβ)  ≥  50 Å (red symbols and lines) and EW(Hβ)  <  50 Å (blue symbols and lines).