  Connecting the observed rest-ultraviolet (UV) luminosities of high-$z$
galaxies to their intrinsic luminosities (and thus star formation rates)
requires correcting for the presence of dust. We bypass a common
dust-correction approach that uses empirical relationships between infrared
(IR) emission and UV colours, and instead augment a semi-empirical model for
galaxy formation with a simple -- but self-consistent -- dust model and use it
to jointly fit high-$z$ rest-UV luminosity functions (LFs) and colour-magnitude
relations ($M_{\mathrm{UV}}$-$\beta$). In doing so, we find that UV colours
evolve with redshift (at fixed UV magnitude), as suggested by observations,
even in cases without underlying evolution in dust production, destruction,
absorption, or geometry. The observed evolution in our model arises due to the
reduction in the mean stellar age and rise in specific star formation rates
with increasing $z$. The UV extinction, $A_{\mathrm{UV}}$, evolves similarly
with redshift, though we find a systematically shallower relation between
$A_{\mathrm{UV}}$ and $M_{\mathrm{UV}}$ than that predicted by IRX-$\beta$
relationships derived from $z \sim 3$ galaxy samples. Finally, assuming that
high $1600 \r{A}$ transmission ($\gtrsim 0.6$) is a reliable LAE indicator,
modest scatter in the effective dust surface density of galaxies can explain
the evolution both in $M_{\mathrm{UV}}$-$\beta$ and LAE fractions. These
predictions are readily testable by deep surveys with the James Webb Space
Telescope.
