They must be turning infrared intensity at various frequencies to (r,g,b) in a way that longer wavelengths contribute to the r channel and shorter wavelengths to the b channel because they say "more distant galaxies" look redder. See
since all space is expanding, isn’t it the case that more distant galaxies are moving away from us faster than more local objects? And therefore would be redshifted as a result?
https://ned.ipac.caltech.edu/level5/March02/Sarazin/frames.h...
for some explanation of how the gas gets so hot