This is 75 nearby galaxies imaged by the Spitzer Space Telescope. They're ordered into a Hubble tuning-fork. Normally, you could observe that elliptical galaxies (grouped to the left) are redder and spirals (grouped to the right) are bluer. What use can see on this image is quite the opposite. However this color difference is something that can be observed in the optical wavelenghts. Since Spitzer is not an optical, but an infrared telescope, they created color composites of their data to show us. The galaxies in this poster are three-color composites where blue depicts the galaxies at a light wavelength of 3.6 microns, while 8.0 microns is green, and 24 microns is red. Therefore, on these images red lumps show clouds of warm dust and gas heated by radiation from newborn stars (glowing in infrared) - a characteristic of spiral galaxies.
Read the Spitzer group's article.