  The explanation for the existence of an excess population of faint blue
galaxies (FBGs) has been a mystery for nearly two decades, and remains one of
the grand astronomical issues to date. Existing models cannot explain all of
the observational data such as galaxy number counts in the optical and infrared
passbands and the redshift distributions of galaxies. Here, by modelling the
morphological number counts derived from the Hubble Space Telescope, as well as
the number counts in optical and infrared passbands, and the redshift and color
distributions of galaxies obtained from ground-based observations, we show that
the `FBG problem' cannot be resolved if elliptical galaxies are assumed to have
formed in an instantaneous burst of star formation at high redshift with no
subsequent star formation events, which is just the conventional scenario for
formation and evolution of ellipticals. There exist great discrepancies between
the observed $B-K$ color distribution and the predicted distribution for
ellipticals by such a pure luminosity evolution (PLE) model in the context of
the conventional scenario. Neither can the mild evolution (i.e., the star
formation events have lasted for a longer time than those of the instantaneous
burst and passive evolution since the formation of galaxies) for ellipticals be
accepted in the context of PLE assumption. The introduction of dust extinction
also cannot save the PLE models. This conclusion holds for each of the three
cosmological models under consideration: flat, open and $\Lambda$-dominated.
Hence, our investigation suggests that PLE assumption for elliptical galaxies
is questionable, and number evolution may be essential for ellipticals.
