Galaxies have different appearances depending on the wavelength of light that they are observed with.
Elliptical galaxies and the bulges of spiral galaxies appear red at optical wavelengths, but in the
infrared their starlight is blue.
The disks of spiral galaxies appear blue at optical wavelengths (due to the blue, massive stars forming
in the spiral arms) but red at infrared wavelengths (due to the strong PAH emission).
The Spitzer's Hubble Atlas of Galaxies (SHAG) project has compiled a representative census of approximately
100 nearby galaxies, spanning all different galaxy luminosities and morphological types.
Spitzer Reveals What Edwin Hubble Missed