Why do spiral arms have a blue color?

Spiral arms are waves of enhanced density of stars and gas in the disk of a galaxy. Stars are widely separated and are not individually affected by the greater crowding in a spiral arm but gas clouds are much larger and, relatively, less widely separated. Thus they suffer more crowding in a spiral arm and the resulting collisions and higher densities are believed to lead to a high rate of new star formation. Any massive stars that are formed will be bluer and brighter than lower mass stars but will live for a much shorter time. Such stars do not have time to diffuse out across the galactic disk and therefore trace the location of the spiral arms, which consequently appear blue.