  Since the Gaia data release 2, several works were published describing a
bifurcation in the observed white dwarf colour$-$magnitude diagram for
$\mbox{$G_{\mathrm{BP}}$}-\mbox{$G_{\mathrm{RP}}$} > 0$. Some possible
explanations in the literature include the existence of a double population
with different initial mass function or two distinct populations, one formed by
hydrogen$-$ and one formed by helium$-$envelope white dwarfs. We propose
instead spectral evolution to explain the bifurcation. From a population
synthesis approach, we find that the spectral evolution occurs for effective
temperature below ${\simeq}11\,000\,\mathrm{K}$ and masses mainly between
$0.64\,\mathrm{M}_\odot$ and $0.74\,\mathrm{M}_\odot$, which correspond to
around $16$ per cent of all DA white dwarfs. We also find the Gaia white dwarf
colour-magnitude diagram indicates a star formation history that decreases
abruptly for objects younger than $1.4\,\mathrm{Gyr}$ and a top-heavy initial
mass function for the white dwarf progenitors.
