The reason in this case is their lovely tobacco-brown dials, and the history behind them. It’s a color Panerai would’ve hated seeing when they began making watches with Rolex in the 1930s. Their brass dials were black and meant to stay that way.
To their dismay, the Florentine house would soon find out their patented radium-based powder (Radiomir), which so successfully lumed the hands and indexes of their diving instruments, would later turn them brown.
That’s exactly what happened to one of its most famous watches, reference 6154. Nicknamed the “Egiziano Piccolo,” it was commissioned by the Egyptian Navy in 1954, specifically because they required the properties of Radiomir for underwater use.