[thaumasiotes]

> What makes a Wordle square yellow? The answer is not so simple

Well, I would say the question is poorly posed. The answer to the question as posed is simple as can be if the guess contains no duplicate letters. But if the guess does contain duplicate letters, the question as posed doesn't necessarily have any answer. It may - if all duplicates should be yellow, or if some of them should be green while all the rest are yellow, there's only one valid way to color the guess. But in the general case, you have some number of green squares, some number of yellow squares, and some number of dead squares, and you can swap yellow for dead without affecting the validity of the coloring.

So the question you should be asking is not "should this square be yellow?" but rather "how many yellow squares should this guess receive?". Then you can just choose appropriate squares to color yellow arbitrarily.