Why do spiral galaxies often appear blue?

The new stars that form mostly faint cool (and thus red) low mass stars, but with a small number of bright hot (and thus blue) high mass stars. High mass stars are much (MUCH) brighter than low mass stars, so this small number of high mass stars outshines the low mass stars, making the whole bunch of stars appear blue. read more

The blue, if you mean blueshift verses redshifted is an indicator of what the object/galaxy is doing in relation to the observe at the time the light left the observed/object/galaxy. read more

There are three different types of galaxies at hand, and they are often distinguished by its shape that is either elliptical, spiral, or irregular in formations. As a general rule, the shape of the galaxy is also defined by its color wherein elliptical galaxies appear to be red while spiral galaxies appear to be blue. read more

Spiral arms are 'density waves' of stars that rotate around the galaxy. read more