  As a means of better understanding the evolution of optically selected
galaxies we consider the distribution of galaxies within the multicolor space
$U$, $B_J$, $R_F$ and $I_N$. We find that they form an almost planar
distribution out to $B_J =22.5$ and $z<0.3$. The position of a galaxy within
this plane is dependent on its redshift, luminosity and spectral type. While in
the original $U$, $B_J$, $R_F$ and $I_N$ space these properties are highly
correlated we can define an optimal rotation of the photometric axes that makes
much of this information orthogonal. Fitting the observed spectroscopic
redshifts with a quadratic function of the four magnitudes we show that
redshifts for galaxies can be estimated to an accuracy better than $\Delta z
=0.05$. This dispersion is due to the photometric uncertainties within the
photographic data. Assuming no galaxy evolution we derive a set of simulated
galaxy fluxes in the U, J, F and N passbands. Using these data we investigate
how the redshift is encoded within the broadband magnitudes and the intrinsic
dispersion of the photometric-redshift relation. We find that the signal that
defines a galaxy's photometric redshift is not related to specific absorption
or emission lines but comes from the break in the overall shape of the galaxy
continuum at around 4000 \AA. Using high signal-to-noise photometric data we
estimate that it is possible to achieve an intrinsic dispersion of less than
$\Delta z =0.02$.
