  We present evolutionary synthesis models for both components and add their
spectra in various proportions to obtain the full range of local galaxies'
B-band bulge-to-total light ratios. Bulge star formation is assumed to occur on
a short timescale of $10^9$ yr, disk star formation proceeds at a constant
rate. Models are presented for three scenarios: bulges and disks of equal age,
old bulges and delayed disk star formation, and old disks with subsequent bulge
star formation. We quantitatively show that bulge-to-total ($=$ B/T) light
ratios for local galaxies increase significantly from U through I-bands with
the rate of increase slightly depending on the relative ages of the two
components. Hence, simultaneous decomposition of galaxy profiles in several
bands can give direct information about the relative ages of bulges and disks
and help constrain galaxy formation scenarios. The redshift evolution of
B/T-ratios in various bands U, B, V, I, H is calculated accounting both for
cosmological and evolutionary corrections. I- and H-bands show the smoothest
redshift evolution and, hence, are best suited for a first order comparison of
galaxies over the redshift range from ${\rm z=0}$ to ${\rm z \gta 1}$ but still
are shown to increasingly overestimate B/T ratios at higher redshifts. This is
a robust result irrespective of the respective ages of the bulge and disk
stellar components and implies that the scarcity of bulge-strong systems at
${\rm z \geq 0.8}$ reported for HDF and Hawaiian Deep Field galaxies is further
enhanced.
