I hadn’t noticed one didn’t have a halo! Thanks for pointing that out. Judas did play a role but the story doesn’t end well for him. He was also accused of stealing money from the group before this moment [0]. I think having a visible halo is kind of funny in general but giving one to all but Judas is actually a solid sense of humor from the artist.
Walking through an art museum a few years ago, I was struck with how many depictions of Jesus had red hair. Once you start noticing it, it’s tough to miss... even though the chances of him looking like that is likely almost nil.
Having someone who looks like your local populace, or what your local populace aspires to be, could help with people embracing or identifying with the person in the picture. This strategy was employed in a variety of religious paintings, and even in this one a little, such as the fava beans. Here’s a version in Peru that incorporated the local cuisine of guinea pigs and corn beer: https://www.atlasobscura.com/places/guinea-pig-last-supper
0: https://biblehub.com/john/12-6.htm
Edit: typo