Final answer:
The reddest galaxies in artificially colored infrared images are most likely moving away from us the fastest, as indicated by redshift which stretches light to longer wavelengths.
Step-by-step explanation:
In assigning artificial colors to infrared images of galaxies, the color red represents a wavelength of 1.60 μm. This longer wavelength may indicate redshift, which is a key concept in astrophysics that involves the stretching of light to longer wavelengths as objects in space move away from the observer. Since we know that more red galaxies are indicative of an older star population and potentially longer wavelengths due to redshift, the correct answer to why the reddest galaxies would appear so is that they are moving away fastest. This is because, according to the Doppler effect for light, objects moving away from us have their light stretched to longer (redder) wavelengths.