Because of the younger stars in the disk, it looks white or blue when we take images. The bulge, with older stars, will look red or yellow. Also lots of dust, ionized gas, neutral atomic gas, and molecular gas in the disk. Mostly older, evolved stars so elliptical galaxies look more red than spiral galaxies. Very little dust or neutral and molecular gas, no current star-formation.
Irregular galaxies have no particular shape.
Our companion galaxies, the Small and Large Magellanic Clouds, are irregular galaxies. Often, but not always, very rich in gas and dust with lots of active star-formation. Everything we discussed about the Milky Way holds for spiral galaxies in general. Size of the central bulge compared to the size of the disk. Number of arms.