  Superorbital periods that are observed in the brightness of Be/X-ray binaries
may be driven by a misaligned and precessing Be star disc. We examine how the
precessing disc model explains the superorbital variation of (i) the magnitude
of the observed X-ray outbursts and (ii) the observed colour. With
hydrodynamical simulations we show that the magnitude of the average accretion
rate on to the neutron star, and therefore the X-ray outbursts, can vary by
over an order of magnitude over the superorbital period for Be star spin-orbit
misalignments $\gtrsim 70^\circ$ as a result of weak tidal truncation. Most
Be/X-ray binaries are redder at optical maximum when the disc is viewed closest
to face-on since the disc adds a large red component to the emission. However,
A0538-66 is redder at optical minimum. This opposite behaviour requires an
edge-on disc at optical minimum and a radially narrow disc such that it does
not add a large red signature when viewed face-on. For A0538-66, the
misalignment of the disc to the binary orbit must be about $70-80^\circ$ and
the inclination of the binary orbit to the line of sight must be similarly
high, although restricted to $<75^\circ$ by the absence of X-ray eclipses.
