  The stellar and dust content of spiral galaxies as function of radius has
been investigated using near-infrared and optical broadband surface photometry
of 86 face-on spiral galaxies. Colors of galaxies correlate with the
azimuthally averaged local surface brightness both within and among galaxies,
with the lower surface brightness regions being bluer. The colors formed from
different passband combinations correlate strongly indicating that they
probably arise from the same physical process.
  A 3D radiative transfer model was developed to calculate the effect of dust
absorption and scattering on the luminosity and color profiles of galaxies.
Stellar synthesis models were used to investigate the effects of the star
formation history and the metallicity on the broadband color profiles.
Combining all optical and near-infrared data shows that the color gradients in
this sample of face-on galaxies are best explained by a combined stellar age
and metallicity gradient across the disk, with the outer regions being on
average younger and of lower metallicity. Dust reddening probably plays only a
minor role, as the dust models cannot produce reddening profiles that are
compatible with the observations.
  The observed color differences implicate substantial M/L_lambda differences,
both within galaxies and among galaxies. The variations are such that the
``missing light'' problem derived from rotation fitting becomes even worse.
Late-type galaxies (T>=6) have lower metallicities and are often of younger
average age than earlier types and have therefore an entirely different
M/L_\lambda in most passbands. The near-infrared passbands are recommended for
studies where the M/L_\lambda ratios should not vary too much.
