Or because they liked the colours for aesthetic reasons haha.

Also choosing a cloth colour has nothing to do with thermal mass and everything to do with absorbtivity/emissivity/reflectivity aka material properties affecting radiative heat transfer.

In any case, shirt colour has a small effect on temperature, maybe a 5°C (at most) difference between white and black, according to some studies. So unless you're really chasing the most optimal clothing, it's best to just wear what makes you happy.