  The transformation of late-type galaxies has been suggested as the origin of
early-type dwarf galaxies in galaxy clusters. Venhola et al. analysed
correlations between colour and surface brightness for galaxies in the Fornax
cluster binned by luminosity or stellar mass. In the bins with $M_\star<10^8
{\rm M}_\odot$, the authors identified a correlation of redness with fainter
surface brightness and interpreted it as a consequence of the quenching of star
formation by ram pressure stripping in the dwarf galaxies. We carry out a
corresponding analysis for the Virgo cluster and find great similarities in
these correlations between surface brightness and colour for the two clusters,
despite expected differences in the strength of the ram pressure. Furthermore,
we extend the analysis to a wider range of optical colours for both clusters
and contrast the results with expectations for fading and reddening stellar
populations. Overall the slopes of the surface brightness-colour relations are
consistent with these models. In addition the sizes of the early- and late-type
galaxies at these low masses are comparable. These two results are compatible
with a transformation scenario. However, when analysing early- and late-type
galaxies separately, the consistency of the slope of the surface
brightness-colour relations with the model expectations for fading and
reddening stellar population applies only to the late types. The lack of this
imprint for the early-type dwarfs calls for some additional explanation, for
which we discuss several possibilities. Finally, the Virgo cluster is an
atypical cluster with a low fraction of quiescent early-type galaxies at all
galaxy masses despite its large cluster mass. (abridged)
