Why is this in ATM? It looks like reasonable questions and answers stuff.

Let's get the chronology straight, at least as I understand it according to the most up to date theory.

The galaxies have been around for some billions of years. We see the nearby ones as they were very recently, and the more distant ones as they were in earlier stages of their evolution.

Star formation rates were generally higher in the earliest stages, and massive, brilliant blue stars of types O and B are believed to have been more abundant. Thus I would expect to see more blue light in the most distant ones. It will be redshifted, but the spectral lines will identify it as intrinsically blue. Redshift alone should not make the blue light unobservable.

If the opening round of star formation had been all there was, there would be no remaining blue stars to be seen in nearby galaxies. The fact that some galaxies, including our own, have an impressive sprinkling of them indicates that episodes of star formation have occurred in recent times. Collisions and mergers can provide the necessary perturbations to start new episodes in previously quiescent interstellar gas and dust.

Other galaxies, which have insufficient remaining gas for such star formation, will have only "later" type main sequence stars, red giants, and the dim or dark remnants of the primordial blue stars. These are the ones that are described as old and red.

I don't think the nearby blue galaxies are younger overall. They just have a greater abundance of star-forming capability. I think the SPACE.com writer used some poor choices of words. I am skeptical when a writer uses gussied up figurative language such as "sexy" in such a context. I would rather hear more from Mr. Martin directly concerning the intermediate stages of galactic evolution.