What makes galaxies different colors?
Answers
The colour of a galaxy is determined by its composition of elements, that are predominant. Also the average temperature of the Galaxy can determine its colour. The stars in the galaxies emit light of continuous spectrum. When certain elements, that are predominant, absorb the photons from the stars. The electrons will get excited and jumps to a higher level. When it jumps back to the lower level it releases the energy as photons that emits light of certain wavelengths that are particular to its element. If the average temperature of a galaxy is hot enough to emit a radiation towards the UV spectrum, the colour will be blue, violet, green. If the temperature is not hot, it will be towards the infrared, that is red, orange, yellow. Also when a galaxy is closer to us it appears blue, and if it is away from us it appears red. Hope this helps.
The various colors in a galaxy (red bulge, blue disks) is due to the types of stars found in those galaxy regions, called its stellar population. Big, massive stars burn their hydrogen fuel, by thermonuclear fusion, extremely fast. Thus, they are bright and hot = blue.