  Our recent multi-band photometric study of the colour width of the lower main
sequence of the open cluster M37 has revealed the presence of a sizeable
initial chemical composition spread in the cluster. If initial chemical
composition spreads are common amongst open clusters, this would have major
implications for cluster formation models and the foundation of the chemical
tagging technique. Here we present a study of the unevolved main sequence of
the open cluster M38, employing Gaia DR3 photometry and astrometry, together
with newly acquired Sloan photometry. We have analysed the distribution of the
cluster's lower main sequence stars with a differential colour-colour diagram
made of combinations of Gaia and Sloan magnitudes, like in the study of M37. We
employed synthetic stellar populations to reproduce the observed trend of M38
stars in this diagram, and found that the observed colour spreads can be
explained simply by the combined effect of differential reddening across the
face of the cluster and the presence of unresolved binaries. There is no need
to include in the synthetic sample a spread of initial chemical composition as
instead necessary to explain the main sequence of M37. Further photometric
investigations like ours, as well as accurate differential spectroscopic
analyses on large samples of open clusters, are necessary to understand whether
chemical abundance spreads are common among the open cluster population.
