Even the best judge eventually meets a case he cannot crack. Two witnesses contradict. A motive stays buried. A theft leaves no trail. The Targum Pseudo-Jonathan on Exodus 28:15 says God anticipated this failure and built a remedy into Aaron's vestments. The choshen mishpat, the breastplate of judgment, was the tool by which are made known the judgments that are hidden from the judges.
The Targum goes further. The breastplate also announced the order of the victories of their battles. Before Israel went to war, the high priest could consult the stones and learn whether the campaign would succeed. And when a judge had ruled wrongly, the breastplate made amends that are made for the judges — a cosmic correction for the inevitable courtroom failures of mortal men.
The artistry matched the function. The breastplate used gold, hyacinth, purple, crimson, and fine linen twined, the same palette as the ephod itself. The Sages taught that truth wears finery. The tool that sees through lies had to be made with more care than the lies themselves.