  We present a study of observed Type II supernova (SN~II) colours using
optical/near-infrared photometric data from the \textit{Carnegie Supernovae
Project-I}. We analyse four colours ($B-V$, $u-g$, $g-r$, and $g-Y$) and find
that SN~II colour curves can be described by two linear regimes during the
photospheric phase. The first ($s_{\rm 1,colour}$) is steeper and has a median
duration of $\sim 40$ days. The second, shallower slope ($s_{\rm 2,colour}$)
lasts until the end of the "plateau" ($\sim 80$ days). The two slopes correlate
in the sense that steeper initial colour curves also imply steeper colour
curves at later phases. As suggested by recent studies, SNe~II form a
continuous population of objects from the colour point of view as well. We
investigate correlations between the observed colours and a range of
photometric and spectroscopic parameters including the absolute magnitude, the
$V$-band light-curve slopes, and metal-line strengths. We find that less
luminous SNe~II appear redder, a trend that we argue is not driven by
uncorrected host-galaxy reddening. While there is significant dispersion, we
find evidence that redder SNe~II (mainly at early epochs) display stronger
metal-line equivalent widths. Host-galaxy reddening does not appear to be a
dominant parameter, neither driving observed trends nor dominating the
dispersion in observed colours. Intrinsic SN~II colours are most probably
dominated by photospheric temperature differences, with progenitor metallicity
possibly playing a minor role. Such temperature differences could be related to
differences in progenitor radius, together with the presence or absence of
circumstellar material close to the progenitor stars.
