  Radial colour gradients within galaxies arise from gradients of stellar age,
metallicity and dust reddening. Large samples of colour gradients from
wide-area imaging surveys can complement smaller integral-field spectroscopy
datasets and can be used to constrain galaxy formation models. Here we measure
colour gradients for low-redshift galaxies (z<0.1) using photometry from the
DESI Legacy Imaging Survey DR9. Our sample comprises ~93,000 galaxies with
spectroscopic redshifts and ~574,000 galaxies with photometric redshifts. We
focus on gradients across a radial range 0.5 Re to Re, which corresponds to the
inner disk of typical late type systems at low redshift. This region has been
the focus of previous statistical studies of colour gradients and has recently
been explored by spectroscopic surveys such as MaNGA. We find the colour
gradients of most galaxies in our sample are negative (redder towards the
centre), consistent with the literature. We investigate empirical relationships
between colour gradient, average $g-r$ and $r-z$ colour, $M_r$, $M_\star$, and
sSFR. Trends of gradient strength with $M_r$ ($M_\star$) show an inflection
around $M_r\sim-21$ ($\log_{10} \, M_\star/\mathrm{M_\odot}\sim10.5$). Below
this mass, colour gradients become steeper with increasing $M_\star$, whereas
colour gradients in more massive galaxies become shallower. We find that
positive gradients (bluer stars at smaller radii) are typical for galaxies of
$M_{\star}\sim10^{8}\,\mathrm{M_\odot}$. We compare our results to age and
metallicity gradients in two datasets derived from fits of different stellar
population libraries to MaNGA spectra, but find no clear consensus explanation
for the trends we observe. Both MaNGA datasets seem to imply a significant
contribution from dust reddening, in particular, to explain the flatness of
colour gradients along the red sequence.
