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.
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