A galaxy’s color is due to the combined color of its stars.
A galaxy’s color also tells us whether star formation
occurred recently, within that galaxy. A galaxy with lots
of recent star formation has many young, blue stars
and will appear blue. After the short-lived blue stars
end their lives, the cooler yellow to red stars continue to
shine. The galaxy will then appear redder, indicating that it hasn’t produced new
stars for awhile.
Just as a galaxy with mostly young stars appears blue, and a galaxy with mostly
old stars appears red, galaxies with stars of varying ages may appear to be a
combination of colors. For example, a galaxy with some young and some old stars
may be a combination of blue and red.